If flying out of STT, depending on departure, may want to extend trip 1 day with last night spent in STT.

Check out all options including flying to San Juan (SJU) and perhaps booking a Cape Air or Seaborne flight from there to Beef Island . Likely to get better deal on booking separately and consider Cape Air Commuter packages (books of 10 tickets) with each roundtrip requiring 2 tickets. (if extra commuter tickets, likely to be able to sell easily on this site....( I have read that you can book flights before buying book as they expire 1 year after purchase). With a group of 6-8 people, Cape Air is likely to hold for departure .

check all your options. Consider costs of ferries, extra taxis, and hotel if needed.