... I've been practicing the G, C, and A scales along with some improvising. ...
For a double bassist in a bluegrass jam, knowing how to read the guitar chords as someone might be playing them is extremely valuable. Knowing Nashville notation and hand symbols also helps. And being able to play and improvise by ear in bluegrass jams are essential skills... But these are all apex skills that can take some time to learn. Initially going to a few bluegrass jams without an instrument and just watching and listening to the double bassist is extremely helpful. Joining a "slow jam" or early beginner's jam at first would probably be the easiest way to start out.
You're soon going to want to be able to play in more keys than G, C and A on double bass. In beginner+ to intermediate to high level bluegrass jams it's not at all unusual to need to play in A, Bb, B, C, D, E, F and G. I'd suggest being able to provide simple and comfortable double bass accompaniment at least for the 1, 2, 4, 5 and 7 chords in each of these keys, plus in their relative minor keys. Knowing the same fundamentals for every single key is a good thing, but may not be immediately required in bluegrass jams. Just like in a band, the vocalists in a jam have the right to name the keys for the songs they lead sing in, so that can happen in literally any key.
Big practice on intonation and rhythm fundamentals is important. Bowing really helps improve intonation, but if you're listening carefully, pizz will also help. Regarding rhythm, keep in mind, if you're the bassist in a jam, you're the person who experienced people will depend on to support the tempo and meter, so it's really important. Practicing with a metronome and/or a backing track program (like Strum Machine for bluegrass) can dramatically help improve rhythm.
Generally, as has been mentioned, don't get too fancy with playing, especially until your fundamental skills are sufficient for jamming. But don't hold yourself back to a point where you get bored, either...
When you can do them well, walks in explicitly the right places are often ok, but it takes time to learn where those right places are, so be very lean and gentle with them -- as has been expressed earlier in this thread, even when you're doing walks tastefully and skillfully, some jams may be more "walk tolerant" than others.
Similarly, you might also want to pick up some simple slap technique, it's often appreciated and can help tastefully reinforce rhythm -- in some bluegrass jams.
Also, work toward being able to take a solo double bass break when asked to, but don't expect to be asked to take a break all the time, or even sometimes in some bluegrass jams.
Lastly, keep in mind that most bluegrass jams have only one double bassist at any given time. You'll have to be capable of the job, plus, you'll have to wait for your turn to play.
This all sounds pretty complicated, and really, bluegrass jams actually are pretty complicated. But bluegrass jams are lots of fun too, so be relaxed, listen and learn, and enjoy your participation in them.