This is a simple little houserule to extend the rules for grappling and "Help" actions in combat. 
Simply put, if you are grappling a creature, you can use a bonus action or a reaction to help an ally land an attack on that creature. 
The rule has 3 basic variants: bonus actions only, reactions only, and both allowed.
You can also choose to allow a bonus action or a reaction, but not both in the same round if you like. 

This is a pretty useful little bonus, but it's limited enough to not be too potent (1-2 times per turn round, must be grappling, advantage can't stack, only benefits attacks, requires a free hand)

On the narrative side, what this represents is restricting the target's movement to make them easier to hit, pulling a shield aside, or tugging them off balance.
This also makes sense when grappling an invisible enemy.

What do y'all think of this rule, would you consider using it at your tables?

You said one or the other in a round, but then said 1-2 times per turn. Since there is a variable number of turns in a round, I’m not sure what exactly you mean. Also, you haven’t actually explained how it’s supposed to work. Finally, you wrote that “advantage can’t stack,” but advantage never stacks anyway. Could you maybe clarify what precisely you mean?

1-2 times per round, I meant. Depending on which variant of the rule you use. The only variant that allows you to use it 2 times in a round is the one that lets you use either an bonus action or a reaction and doesn't stop you from using both in the same round. Every other variant of the rule is limited to once per round.
The point is it's not a free thing you can use a whole bunch of times, it's limited and prevents you from doing other things with your bonus/reaction to do it. Saying "advantage doesn't stack" was pointing out one of the limitations of this rule, because people are more likely to implement this rule at their tables if they realize it's not a powerful enough benefit to upset the game balance.
How do you feel about the houserule in general? I don't mind if you dislike it.

I honestly can’t say because you haven’t stated what it actually does mechanically. What does “helping an ally land an attack on a creature you are grappling” (paraphrased) actually mean? Does it mean granting them advantage on their next attack? In that case it’s something you can already do simply by taking the Help, only differences being that the Help action requires a whole action unless you’re a hobgoblin from Mordenkainen Presents, you don’t need to be grappling the target creature to do it, and you have other options on what to do when helping. So if you do mean simply granting advantage, then all you’re really saying is they can use part of the Help action as a bonus action or reaction, but only while grappling a creature and it only works against that creature. (If you are intending some other benefit besides advantage, you gotta explain it better.)

In that case, I don’t really see anything wrong with it or anything, but what the context? Is this just supposed to be a general rule anybody could use, or is it part of a feat or subclass feature or something?

Yes, to clarify I was refering to the help action and granting advantage on an ally's attack. Yes it's supposed to be a general houserule that anyone can use.

The entire purpose of this house rule is that if you are grappling a creature, you can grant advantage on an ally's attack (combat help action) against that creature without having to use your main action to do so. It follows the normal rules for combat help actions, and changes nothing except for making it easier to do on a creature that you are grappling.

I think the main reason I failed to explain clearly is because it's actually a very simple change, the one line 
"Simply put, if you are grappling a creature, you can use a bonus action or a reaction to help an ally land an attack on that creature."
tells you most of what you need to know. I'll add [action] tags to the "help" to make it a bit clearer.