d2OKC
Explorer
As a DM, I like to give the PCs a common thread and then tell them where they're going to start before we sit down to play, and it's up to the player how their character got there. And generally, I ask them to stick with similar alignments just to make things easier.
For example, in my current campaign, the common thread was an NPC who was going to be executed. I asked each of the players to give me a reason why their character hated the soon-to-be-dead NPC and then told them they'd all be at the hanging when the campaign started.
During the execution, there was an attack, and the PCs were the only ones to stay behind and fight off the attackers. Afterwards, they found clues connecting the deceased NPC and the attackers and they learned they all had reason to dislike the guy, and that was enough for them to agree to investigate this connection together. I barely had to do any work outside of setting up the encounter and then running the combat.
They've since spent a lot of time adventuring together, and have learned to trust each other in-game, which keeps them together.
I know the OP didn't ask for ways to avoid it, but I thought it might be helpful anyway.
For example, in my current campaign, the common thread was an NPC who was going to be executed. I asked each of the players to give me a reason why their character hated the soon-to-be-dead NPC and then told them they'd all be at the hanging when the campaign started.
During the execution, there was an attack, and the PCs were the only ones to stay behind and fight off the attackers. Afterwards, they found clues connecting the deceased NPC and the attackers and they learned they all had reason to dislike the guy, and that was enough for them to agree to investigate this connection together. I barely had to do any work outside of setting up the encounter and then running the combat.
They've since spent a lot of time adventuring together, and have learned to trust each other in-game, which keeps them together.
I know the OP didn't ask for ways to avoid it, but I thought it might be helpful anyway.