RPGMemes

Ngl, this has never been a problem for multiple sessions for me. As a player or DM.

As a player, I show up willing to play characters that will work with a group, even if they don't trust them. Trust isn't necessary to work together.

As a DM I remind all players of that fact before they roll one up. If they don't have an idea on how their character would manage that, I'll give them ideas.

Yeah, you'll run into players that just don't get that not every character has to have the same motivation to work with others, or just refuse to play different characters (instead, they try to play the same character with different names). But those are rare. And, so far, I've yet to run into a player that wouldn't take the "look, you don't have to keep playing with us, but give it a try my way and see how it goes, yeah?" talk and give it a fair try.

I've also never had a player quit because of the game not being engaging and fun.

Alone and vulnerable, you are murdered by thieves. Make a new guy for the next game.

This shouldn't be the GMs job btw, players, roleplaying and backstory are YOUR department, write a reason why your character would end up with the others. Work together.

If you don’t have a reason to work with the group, accept that this is a one-shot for you, which may be retcon’d as needed.

The whole We play a game so you have to cooperate together even if role-play wise it makes no sense is a bad practice, May-be not at the point you'll leave the table but definitely a serious sign that the table doesn't function properly.

Luckily, there is a very easy fix Do a session zero, and build a coherent party ab initio, it include in game reason for the party to work together, coherent goals (because when player A wants to abolish the reign of the emperor, and player B wants to defend the emperor you'll have a PvP fight within 3 session) and a meta discussion to have a pallet of skills matching the party's goal (At least in more epic game where you don't want to feel powerless). Almost every RPG published in the last 10-15 years contains an extensive session zero guide and tons of tips to build a relevant party.

If someone wants to play a law priest in a pirate campaign or any other character not fitting the campaign theme or opposing other PCs, it's perfectly OK to tell the no. Obviously if everybody is aligned on some PvP and betrayal the answer may be different, but it's again something to address in session zero.