I think I'm generally farily quick at picking up/following new tunes, and also quite good at knowing very soon whether it's one I can attempt or should leave alone. For me, it's partly a matter of having a reasonable musical memory (at least in the short term), which can cover up for all manner of musical inadequacies in other areas, but practice also plays a big part.

I don't agree with those who say you shouldn't join in until you know it - it may sound ridiculous but I often join in when I don't know the tune, and find I learn it much more quickly than if I just sit and listen. I do listen carefully, and try not to second-guess where the tune or the chords are going.

I'm not sure quite how you cope with new tunes on an autoharp, but with melodeon, I often drop the melody side and just play the basses (chords)until I get a feel for the tune. In many cases the melody just slips in over the top quite naturally once I've got the chords sussed. Having a go at a new tune does mean that if the same tune comes up again in a few weeks, some parts of my brain and fingers will recognise it without having to re-learn it.

But we're all different, and some people do it entirely differently. Melody, rhythm, and chord/harmony all matter, but some instruments - and some players - are more concerned with one of these than the others. I'm sure you'll soon sort out a niche for yourself at local sessions.

Majr