I've heard of people bring this sort of thin up on homebrew forums. I think there end up being a lot of drawbacks to this, unless a lot of equipment is purchased. 15-20 people is a pretty broad base. Free time on the equipment might be tight. If you do a larger "few barrel" system, there might be a hard time getting people to agree on what to brew to split up. Where would the equipment be located? Is rent involved?
Also how far would you go? Just a large brewing system where the contributors to the batch would take home wort? Or would you have a larger place with fermentors, temp controlled walk in, bottling/kegging equipment?
Personally, I wouldn't want to contribute money to a system I wouldn't always get to brew on, that someone else could break, and I ultimately don't get to always decide what to brew. I see this idea working better as a Brew On Premises business operation. For a small fee, people can come and do full all grain batches, ferment and bottle on location, then walk home a few weeks later with ~10 gallons of finished beer.