I think this will be hard to plan in advance but easy to set up once you're here. Just pick one of the main tour / dive operators and do the Hol Chan dive with them (it's pretty shallow the entire time). Ask if they'll add on a private charter for your friends and you'll be all set.

Places to try: Searious, Tuff E Nuff, Amigos del Mar, Chuck & Robbie's, Seaduced, etc. Most have offices on the beach in town so you could walk the strip, pop into half a dozen to find out availability & pricing, and then take your pick.