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Get as many people up for a scene as feels appropriate. Designate one as person as the "spotlighted" performer.

The goal for the spotlighted performer is to do whatever they want. Being present is encouraged, but honestly, just don't worry too much about it. What do you like to do in improv? Do that.

The goal for everyone else in the scene is to help form the story around the spotlighted performer. Depending on how they behave, they might establish a lot of story groundwork on their own. Or they might not. They may speak a lot, or they might not. Your job is to fill in whatever is needed. You can be a strong supporting character, you can play various small roles, it doesn't really matter. Your goal is that, if the audience didn't know what this exercise was, they shouldn't be able to tell an exercise is even going on. It should just be a story like any other.