Hey guys. I've been dealing with some people abandoning the Campaign due to COVID-19 recently, that or some of my long-time players leaving for new job opportunities where their schedules conflict. I have been aiming to bring in new blood to the campaign and give it a renewal. What are some of the ways that you guys go about introducing new PCs into an existing, possibly even longtime campaign.

Any way I can. I mean that, it depends on the situation and the characters. I’m not afraid of “You enter the inn and look around the room. You notice a stranger observing you from a quiet corner.”

Or “As you approach the bend in the road you can hear the sounds of conflict: multiple voices shouting and the sporadic ting of steel on steel. Rounding the last clump of trees you see....” and that’s where they find someone beset by bandits or some such other.

Whatever fits is what I use.

You are allowed, once per campaign, to rescue a prisoner (or prisoners) who then join the party. It is known. 

Otherwise, whatever works. Long lost family member; someone else who is fighting the same BBEG for their own reasons; a happy-go-lucky wanderer joins up because it seems like it’s interesting.

The cheesiest, but easiest way, is to just say, “oh, you’re here now” or “you were always here, don’t you all remember” and just move on from there. 

Take inspiration from films and video games, how did Han join the party, how did Madmartigan, how does Shepherd get party members?

people band together because they need eachother so work out something the party needs from the character and something the character needs from the party

maybe the party needs an expert thief to rob a casino and the thief needs a couple of meat shields to put between them and the enforcers trying to recoup debts.

maybe the party needs a navigator for a sea journey and the navigator needs some protection from pirates on a sea journey

maybe they rescue the new party member from captivity and  they decide they owe the party a life debt

maybe the new PC rescues the party ...

I don’t think there is a wrong answer on folding people in, but I think the coolest way would be to do some 1-on-1 or backstory missions with the new player first, so instead of feeling like they’re just getting dropped in to “well we are all level 10 and already killed the necromancer and are now going to the Abyss to fight this demon, strap in”, they can also have something they were up to previously, and potentially provide a new perspective or information on the main quest. Kind of like a “wait, orc raiders were just attacking my city and making off with all kinds of magical ingredients and artifacts, do you think it’s related to the string of kidnappings happening in the castle near you?” 

If you do a 1 on 1 you should make the party they are joining the BBEG

Any number of ways, it really depends whats happening in your plot.
I have:
Had one being a hermit who heard the party fighting and came to help.
Had one being a prisoner the party rescued
Had one being a captain who had just been rescued after his ship sank and had been dropped off at the small port town my players were in.
Had one be introduced as a friend of a high level NPC who needed aid.

If you tell us what is currently happening in your game, we may be able to give you a more detailed idea.

In my current campaign, the three original players got into some trouble with goblin raiders attacking farms and trade routes, so they were personally invested in getting back at the raiders. After some escalation and some hints of more behind this bold behavior, I could tie in the new fourth player, who had a Soldier background, by having a small intro into the situation via pre-session phone call. He was asked by his superior (he is a Monster Hunter Ranger) to lend the local Guardmaster in that region of the attacks a hand, because they knew each other from a past war. No official order was possible, because no one in the nobility believes that there is a real threat going on with these goblins.

Something like this can be build up together with the new player in a x.5 session before going into session x+1.

I'll be honest, sometimes I just fiat it. "This is Sarah. She's the new Jeff. Treat her as if she's always been here."

Almost any "new player" storyline requires entirely too much railroading and monologuing. The players basically don't get to play their own game for at least a half hour while you shoehorn in an intro quest. Because things HAVE to happen a certain way for the player to enter the game, it means you spend a ton of time dragging the players to a plot. AND THEN you have to deal with introductions all over again. And I'll be honest, introductions are the worst. You can't force new player unity, and you never know if a new character (especially if they're played by a new player) is going to behave well with the party. For all you know, the new character is gonna be all "I have no reason to trust you people" and insist on wandering off on their own.

People will argue that fiat isn't cinematic, but neither is stopping all plot progress to introduce new characters to the team. Besides, I defy you to name one movie or TV show where half the core characters are replaced with new characters that the audience likes half as much. Meanwhile, there were three separate Becky's on Roseanne, and nobody batted an eyelash.

First new player that joined.. I wrote up a small one shot for him and gave the others characters to play. At the end of where he met up with the other players and they continued from there. 

Three others joined and I had them working for over of the NPCs in the area to investigate the players and report back. Eventually the npc told the new players to stick with them as they had similar goals. 

So there's lots of ways to bring in new players. Just pick what you feel is cool. Talk with your current players as how they would see others joining and see what they come up with.