The range of musicians will be from the sublime to the ridiculous, but go along, put your bass in the car, and at least go and watch what happens. If you find there is no other bass player there make yourself known, you may be on all night. If there is another bass player, ask if you can do a couple of numbers and suggest something you know well, chances are the other muso's will know it.
Most of all, no-one will judge you, go and enjoy it.
Steve