Hello! I would appreciate some advice. I currently DM a group of 5 players, and we play every other weekend. We have done 7 sessions now, and one of my players has only shown up once. It is really frustrating for me to have to rebalance encounters depending on whether or not the player shows up, besides the fact that it is hard to figure out what to do with the player's PC. If you guys have any advice, I would really appreciate it.

This is going to depend on how you and the table want to handle it. Personally I would discuss it with the rest of the players, rather than make a unilateral decision. 

You can start by talking to the player, and asking what is up. Explain that this is making it hard for you. Does this person really want to play? Will they start coming? etc. Depending on the answers, some of the below might be more or less appropriate but, in general once you have talked to the player, your options are....

• Kick the player out of the game, and balance everything for 4 instead of 5 down the line. This can work if you are playing with vague acquaintances, and is very difficult to do if you are playing with close friends/family/coworkers. 
• Have the PC just not be there (sounds like this is what you are doing). This can work mechanically, but if you have to keep coming up with in-story reasons why Jenny the Genasi is never around, or often not around, it will become exhausting. Frankly I don't do this anymore. If someone doesn't show without warning, their character is just not there. Yes, this may mean you have to rebalance on the fly. Welcome to DMing.
• Play the PC as an NPC. You control it and do whatever makes sense, or make die rolls to see what the NPC will do to keep it as objective as possible. This makes more work for you in one sense (RPing an NPC) but will prevent you from having to rebalance things, which is less work in another sense.
• Assign the character to one of the other players, or ask for a volunteer to play it. This is probably the best option for the DM aside from a /kick, but is putting a lot of extra responsibility on one player. You could have the players take turns.

I do a hybrid of the last 2... One of our players has only been in like 3 of 22 sessions, because he has had some serious family emergencies. Knowing what he is going through, I'm not going to kick him or vanish the character. So, I RP the character in noncombat situations, and then one of the players takes him over in combat. Two of the more experienced players take turns with him.

This is really great advice, thank you so much!

Give the character a curse which inflicts them with an entirely absent mind. They become open exclusively to suggestion and will defend themselves but otherwise will stand vacantly for days on end.

The other PC's can give this character instructions (making the characters actions a group decision) and then the DM then acts out that characters basic actions to fulfil their requests.

Then you need to talk to the player and find out what they are doing - are they going to start showing up to games, or are they going to quit?

If they say they will start showing up, write a quest to get the player cured of the curse so they can control them all the time.

Otherwise (they say they aren't sure or their life isn't working with their desires to game) then have their character checked into happy-acres home for the incurably cursed. They wil ltry to cure them, but the party has to carry on without them. If they come back and want to have their old character back, then they can check out and find the party. If they go sketchy again, use your discretion as to whether to kill their character off or have them put back in the home. Depends on whether they give a "I just couldn't be bothered" vibe or a "I really wanted to but a hundred things happened to stop me". Allowances for real life should always be made, especially if they really like their character.

A lot of this depends on the circumstances. Do they have good reasons for not showing up? Do they understand the extra effort it takes to rebalance? Are they giving reasonable notice? Do they actually want to play, or are they just part of a group where everyone else wants to play? All of these will have an effect on how to deal with it.

That said, if they are only going to turn up to one in seven sessions, I would start assuming they are not going to be present. Plan everything for the 4 players who regularly turn up. If the irregular player turns up, keeping some stuff as a backup to throw in if the encounter is too easy isn't that difficult to plan for (an extra enemy or 2 who run in at the last minute).

At our table, we have a rule that everyone progresses at the same rate, but those not present need to offer a description of what they were doing during the missing sessions to "justify" their progression (we're not strict, and everyone enjoys this part at our table). It is possible that this will help with your unreliable player: if they have to describe what they have been doing for the last 3 sessions, coming up with a story of their own which fits, they may start to understand the effort you are putting in.

If you play them as an NPC or another player playing them you must have a plan what to do if the player dies. The absent player must know this is a possibility.

Quote from BioWizard >>

This is going to depend on how you and the table want to handle it. Personally I would discuss it with the rest of the players, rather than make a unilateral decision. 

You can start by talking to the player, and asking what is up. Explain that this is making it hard for you. Does this person really want to play? Will they start coming? etc. Depending on the answers, some of the below might be more or less appropriate but, in general once you have talked to the player, your options are....

• Kick the player out of the game, and balance everything for 4 instead of 5 down the line. This can work if you are playing with vague acquaintances, and is very difficult to do if you are playing with close friends/family/coworkers. 
• Have the PC just not be there (sounds like this is what you are doing). This can work mechanically, but if you have to keep coming up with in-story reasons why Jenny the Genasi is never around, or often not around, it will become exhausting. Frankly I don't do this anymore. If someone doesn't show without warning, their character is just not there. Yes, this may mean you have to rebalance on the fly. Welcome to DMing.
• Play the PC as an NPC. You control it and do whatever makes sense, or make die rolls to see what the NPC will do to keep it as objective as possible. This makes more work for you in one sense (RPing an NPC) but will prevent you from having to rebalance things, which is less work in another sense.
• Assign the character to one of the other players, or ask for a volunteer to play it. This is probably the best option for the DM aside from a /kick, but is putting a lot of extra responsibility on one player. You could have the players take turns.

I do a hybrid of the last 2... One of our players has only been in like 3 of 22 sessions, because he has had some serious family emergencies. Knowing what he is going through, I'm not going to kick him or vanish the character. So, I RP the character in noncombat situations, and then one of the players takes him over in combat. Two of the more experienced players take turns with him.

 I have done the opposite.  I either just kick the player out or rebalance for 4.  I don't worry about an in game reason why Jenny is there  but not active. Or just use the "Chaos Fog" which eats PCs then spits PCs out.  I am more of "THis is a game which has a story" than "This is story telling game."

Thank you all for you answers, they have really helped me figure out what to do!

Glad we helped but... what did you decide to do after all this?

l heard of one group who had a deity that was a giant purple baby that would pick up missing players characters and put them back when the character returned. (LINK)

Quote from BioWizard >>

Glad we helped but... what did you decide to do after all this?

 I am currently texting with the player and they are basically telling me that they don't like to play for 3-4 hours (which I understand but literally everyone else in my group prefers that), and that they have other hobbies that are more important than dnd, plus chores. They don't seem very willing to change their schedule (they are telling me what they have to do, and if they just wake up earlier they could get it done). I think I'm going give them the choice of either dedicating more to dnd or not playing, because its unfair to everyone else in the group.

EDIT: I told them, and they agreed that not playing is the best option for them, since dnd is not their priority.

I have some players that rotate in and out so the group variable is between 4 and 7. There are real life reasons and it isn't a problem. Usually the table comes up with something light hearted as a reason - they went shopping, they're tending the mounts, they're repairing the wagon, etc.

Ramping up isn't so hard. Just have some extra minions show up.

you could make your character a wanderer who has a seperate or secret agenda or just likes to take time to be alone. then the character can leave and then return when the player does.

 I am currently texting with the player and they are basically telling me that they don't like to play for 3-4 hours (which I understand but literally everyone else in my group prefers that), and that they have other hobbies that are more important than dnd, plus chores. They don't seem very willing to change their schedule (they are telling me what they have to do, and if they just wake up earlier they could get it done). I think I'm going give them the choice of either dedicating more to dnd or not playing, because its unfair to everyone else in the group.

EDIT: I told them, and they agreed that not playing is the best option for them, since dnd is not their priority.

Yup -- clearly they were not a good fit. It's not anybody's fault, and hopefully there are no hard feelings.

Thank you so much for the advice! It's all good, they were fine with it.

Honestly if they have not for that many sessions then I would see it that they just dont want to play. D&D is as much a game as it is a hobby that requires a fair amount of dedication. You should talk to them first before giving them the boot. Make it clear that there needs to be some effort put in. Yeah, have a chat with them. If you cannot get ahold of them for whatever reason then the next time they show up you just need to set a standard. Start making encounters for the group who shows up regularly also. I know in my games I would not put up with it. Every one of my players are very clear if they will be missing a game. Lucky for me, this is a rare occurrence. I am fine if a player is going to not be able to make it as long as the let the group know.