Realistically, I don't think 3 or 4 hours is enough time to go to "at least Bellagio and a couple of other towns". You can use ViaMichelin.com to get estimated driving times from Como to other places around the lake, but a tour bus that has to load and unload 30 or 40 people will be much slower than a car. ViaMichelin gives 52 minutes as the estimate for driving from Como to Bellagio, so the round trip on a big bus would probably chew up over 2 hours, and I have no idea how close a large bus could get you to the picturesque part of Bellagio. I suspect Bellagio isn't a suitable place for a drive-by bus tour, but I could be wrong about that.
Bellagio is a ferry ride away from the other towns and fancy villas most often mentioned as sightseeing targets. The ferry rides are fairly short in the mid-lake area, but there's the schedule to consider, and I don't know what you'd need to do to assure all members of your group were able to get on the same ferry.
It will not be simple to maneuver a group of 30 or 40 people through the narrowest (most picturesque?) streets of towns like Bellagio and Varenna.
I wonder whether a private boat trip up to and around the mid-lake area wouldn't be the most practical idea, given your time constraints. You could perhaps stop at one of the villas/gardens, but that would require careful arrangements, because they have timed tickets and I don't know which ones sell enough tickets per time slot to accommodate your group.
You should also research the terrain at any place you want to stop, if there are folks with mobility issues who will be participating.