Good morning,

I'm the kind of person who likes to deal directly with the company so I have no experience with a broker. However, with a large group like yours, it sounds appealing to go from the airport in St. Thomas directly to the boat. That said, there is a charter company in St. Thomas that gets high marks on this forum - CYOA charters. They have very nice boats and I would get a quote from them also.
Finally, I would highly recommend that you collect everyone's share of the charter cost as you go along i.e. deposit and final payment. People as much less likely to drop out and if they do, you aren't stuck with finding replacements or collecting more from those left.
Fred