In a DC Universe game, we had our PCs be relatively new supers who were being trained by a hero known to take new heroes under his wings and train them (Red Tornado, I think). We all basically new each other through him and had worked together in minor ways. Then, when the plot hook for the game (group of baddies are taking over Gotham, and Batman is just out of his league) happened, we decide to go there as a team.

So, common mentor. Or common plot hook/motivation.