Go with your whistle in your pocket and your bass in the car. Go in and you'll soon know if you're confident enough to join in. In my experience someone will soon start talking to you and you'll be able to decide whether that particular session is for you or not. Don't be put off. Ive got very little confidence. I can remember looking at a notice board with a list of singers nights for the local folk club. I stood there thinking that I could never go to anything like that. Then a friend took me along to a session. I couldn't walk in until she arrived. I had to wait on the carpark for her.(Pathetic) Now I can walk into any session in the area and join in. I still have crises of confidence when a song I perform doesnt go that well but I hang in there because since I started going to folk events Ive had the time of my life.