Hi @markmacrae Welcome to @BanjoBen 's forum, Ben is on a family vacation so it may be a few day’s before he responds.
Let me say right off the bat your not alone but you’ve come to the right place to find help.
Whilst your waiting on Ben getting back to you take look at the lessons on banjo backup start with some easy vamping lessons. Once you know a pattern find a tune that you know. Using a mandolin or guitar solo from Ben’s list of mp3 practice your backup at the slowest speed.
Once you get good and comfortable with the chord progression. Look at a few lessons on rolling backup then apply the rolling backup to the tune you’ve been working on. Once you get comfortable doing that try mixing the vamping an rolling patterns.
Playing with others generally means that you are providing accompaniment to their solo spot this also includes singers. YouTube is a good source to find others to practice with. Rather than searching for headliners or specific tunes try searching for bluegrass jams. A lot of your time should be spent LISTENING trying to figure out which KEY they are playing in. Not every tune is played in G at a jam session. Once you figured that out the KEY make a note of the tune title and key. Save the YouTube link to your favourites then with your list you will be ready to practice.
As your listening just play rolls or vamp over the chord progression, most of the time you will get it wrong, don’t get disheartened it’s part of the learning process. The more you listen and try to figure things out the easier things begin to fall in place.
Even though you can play a few tunes I would encourage you to work your way through the Beginners Learning Track as there is a lot of stuff in there that will add to your knowledge and skill level.
Study the two Waypoint’s lessons in the Intermediate section and the Fretboard Geography Course with Alan Munde