So... the discussion has veered from unfriendly folk musicians in pubs to session ettiquette - violations of which result in unfriendly folk musicians.
Would it suffice to say that if you are not well versed in the style of the tune/song being presented you sit tight and appreciate it? If you are well versed, and can lend a bit of support, can you be subtle in requesting to join in? Is the onus on the person desiring to join another in making music on a particular song/tune or should the lead/soloist be put in a position of saying "I'm doing this solo so don't think of joining in or else"?
Finally, my PERSONAL pet peeve, non-performing observers attempting to clap in time with the tune, and not quite being there. (One session I was in, a rather intoxicated fellow managed to clap EXACTLY 1/16 note off the beat - I doubt he could do it when sober.)
Regards -