Group of us traveling together but leaving for home on various flights out of SJU on March 30, 2019. First flight out is 12:40 pm. Carry on luggage only. We are arriving via charter airline from Virgin Gorda that morning. The charter booked us for arrival at 10:10 am. Is that enough time? I'm sure they know what they're doing but checking with my traveltalk peeps. Thanks!

Sounds like enough time.
You will have to clear C&I coming in from VG -- if that is the same C&I you may be able to go directly to the departure area. If you clear C&I at a separate location you will have to pass through again at the main airport entrance before clearing TSA.
For sure have your boarding passes printed and in hand before this all.!!!
Similar to landing by ferry - you must clear into the USA and again later out of the STT point of departure.

I agree, arriving by charter 2.5 hours before first scheduled connecting flight should be fine. The last time I came into SJU on private charter flight from VG we cleared Immigration & Customs at the FBO terminal rather than the I&C location in the main terminal. We had only carry-on and had checked in on line for our SJU/USA Home Airport flight so all we had to do was walk to the main terminal (about a 10 minute stroll) to clear TSA and proceed to the gate for our connecting flight. That whole process took less than 30 minutes.