Ms Lemon, barge into that room and introduce yourself!

People at a singaround are learning and sharing, not "performing". Thus they do not want to annoy the people in the main room with starts and stops and sight-reading-along-with stuff. They are all learning new songs and having a good experience, but they are sensitive that someone in the pub for a drink and nosh and chat with friends don't need to hear all their humble imperfection. Sounds like they were being polite not rude.