Polling, probably - if the majority of group members are bad actors you're fucked.

Not very important, even if generated by a single actor N has not such a big importance. If I were implementing something like this I'd just probably make it -hardcoded-.

If you reaaaallyyyy want to decide on a N on the fly, I'd put a restricction (a<Nx<b) make each participant generate a Nx and then sum then all, -multiply'em If you wanna be hardcore- But I'd be tricky to get it right, for example a party might be able to consistently make N whatever the max value of N is by making their Nx very big -Which, well, I don't really know how it would benefit that party and how would they exploit it-. Maybe using a operation like a XOR on the Nx would be robust enough, and would mitigate the kind of attack that I described above

Tl;dr: you can just have a random party generate it.