I can second Gervase's remarks about the Cardigan Bay Club I was there with him that night. A woman appeared half way through the session clutching a guitar and the organiser announced "Oh good another singer!" and invited her up to sing. Obviously the advice about taking a guitar case along would have helped us out on that occasion but we shouldn't really have to! The organiser's wife did show interest when I sang and asked me about the song afterwards but we were left with the impression tha twe were interrupting their practice session!
At Pennymoor in Devon we make a point of welcoming strangers and all get a song. I remember the first time I went there I was immediately approached and welcomed by Len the MC and asked if I sang. They hold a very friendly singaround with a jam session to start and finish it off. Another friendly session was Bodmin Club where we were welcomed and even given a extra song at the end of the night as visitors!