I agree with most of what other people have said here - that you won't learn without trying, but trying quietly and possibly in private first (if you can persuade a tame melody player to come and play some tunes for you to busk along to) might be a good idea. I guess as a melody player myself I am perfectly happy for people to experiment along with what I'm playing in sessions so long as it done sensitively and they are playing 'with' me not 'against' me. Listen listen listen!
Also, don't forget that all those people who seem to know exactly what they are doing are probably making it up as they go along to a certain extent so don't be too intimidated by them! Most folks are happy to give a few tips as they go along if you ask them nicely.
Webfolk, I think I know why breezy told you to piss off although that was a bit harsh... but saying 'if you don't know, you really shouldn't play.' without some sort of qualifier sounds like you're saying the world is divided into them as can and should, and them as can't & shouldn't. Everybody has to learn somehow!