How big of a group are you going to have?? I've done 80+ people shot and it was not easy.... For one, have an assistant. When you get more than half-dozen or so people together, there will always be some who won't be listening or paying attention. You'd want your assistant to go over there and beat him up. (kidding)

For two, bigger the group, more difficult it will be to ensure everyone is correctly lit. It's much easier if you can line them up under a big tree or something. (that's how I did mine) Mid-day sun on stone stairs will be a problem as it will reflect much more light than people would. You can end up with hot spot if you are not careful.

If you have sun directly lighting faces, sharp distracting shadow can form on faces. (faces are 3 dimensional!) You really don't want to be using reflectors to "touch up" 20 faces at once. (more reasons to use shadowed area) You really don't want back lit situation going either as fill light will be next to impossible.

I don't think choice of film will matter that much. I'd just use whatever you are used to. Same with lens... With larger group, you can even use standard (80mm for MF) lens and not be "weird". You'd be sufficiently away from everybody anyway.

Good luck....