You should give each group the option where and how they want it!

For those who look at you numbly, with no ideas, you can have a couple spots already picked out that have nice-looking background, give them the option of doing a traditional shot where some people are sitting in chairs and others standing behind them, etc.

Having a little variety will make the end result more fun. And definitely take two shots of each pose, as that way, if someone is blinking in one, they might not be in the other.

Be wary of people with glasses - glare can happen. Give them the choice of glasses of or on. Some, like me, will prefer to keep them on, (no one would recognize me sans glasses, anyway) but it's nice to give them a choice!