Hmmm, well. I'd stick to melt & pour. I'd have safety concerns about such a large group of unexperienced folks handling lye. Too many cooks in the kitchen could lead to dangerous results.
You could pre-prepare some soap clay for some of the group to make into decorations/embeds. Some of the group could be in charge of preparing ingredients and keeping the workspaces tidy.
Perhaps splitting them into groups of 3-4 "teams" would be best. One person making the embeds, one person preparing ingredients, one person keeping things neat and clean, and one person doing the blending/pouring.
Here's what I'd do:
Buy a 10lbs block of M&P base, some soap clay, a handful of colourants, any other gear needed. Get a bunch of small boxes to line and use as molds.
Split people into 3-4 person teams, give everyone the same "theme" they must adhere to. Maybe something like "Ocean", "Nature", etc. Adds some fun challenge to it.
Give each team a step-by-step guide.
Let them go wild!
When soap is done and cut, sit down and look at all the creations and discuss.