In general, guitarists are handled about the same as the other folks: You’re expected to at the least, know several chords and be able to play rhythm and change chords pretty smoothly.
Beyond that, it’s a plus if you can offer some songs that you sing, at slow tempos, with easy chords, preferably in G. If you can do any soloing, that’s fine, but it’s not expected. We are mainly about:n1. Getting people comfortable in a jamming situation, following along.n2. Teaching people how to be valuable contributors to a jam session (singing lead, singing harmony, taking solos, leading the group for a song at a time). Some people are ready for this, some not. All are welcome.
My wife Joan is a good rhythm guitar player, and both she and I will give pointers to the various guitarists regarding a good balance of sound, bass runs, etc. Not a lot of specific instrument instruction, but pointers as a person’s playing relates to helping the group sound better.
In sum, you will get a good workout on your guitar, and learn some of the fine points of how to handle your guitar at a jam, building confidence as you go.n
There are a few rhythm guitar players (including Delbert Williams, a very respected California musician), who do use a thumb pick, in the style of some of the earliest bluegrass guitar players, such as Lester Flatt and Carter Stanley. But these exceptions are rare.
I would say a flat pick is not *necessary* for bluegrass, but the standard way the guitar is played in bluegrass does *not* call for fingerpicking at all. Instead, it is typically a strong and clear bass note followed by a quick and clean (not noisy) brushed strum on mostly strings 1, 2, and 3. The idea is to have the low note, and punctuate time, not to “fill” the midrange sound of the ensemble.
Generally, people make the bluegrass guitar sound with a flat pick, but if the grip, etc. are awkward at first, you can try for the same sound with a thumbpick and maybe a single finger pick for the quick strum (not individual notes). Sorry to sound dogmatic about that, but I feel responsible for helping people to understand and learn “the bluegrass way” from me, which they then can use according to their own judgement.