Two options I have personally used. Roatan Air is one. They have a very nice Air Commander that will seat 6. 5 divers and luggage should be no problem. Their website is still advertising $470 each way from Roatan to Utila. Its a great plane and I've been pleased with their service. Another option is the Cessna 206 that is based on Utila. It is primarily used to transport Utila Agressor guests, but can be arranged for other trips as well. They were cheaper than Roatan Air from San Pedro Sula to Utila, so I assume the same is true for Roatan to Utila, but don't actually know. If you'd like that info, PM me and I'll find you the right contact. Candidly, if I had a group going to Utila (and I just got back after a detour to the Galapagos), I wouldn't do anything other than a charter. FWIW.
Cheers.