Maybe make pre-gens that they get to name themselves, with a brief back story? If they are new to the hobby that might work fine, but if they are old-timers, they will probably prefer to make characters from scratch.
If that won't fly, I'd try to present a figure of authority during character creation, stressing that this is someone that the characters revere and trust. That way, you'll have someone that can at least present them with a mission; a mission you can prep for a bit in advance.
But there is no telling what s gaggle of players might do. If they take a left turn, try to roll with it, and if things start to break, you can tell them that you hadn't prepped for what they did. And make sure to have fun!