In sessions, I play guitar (chords) and mandolinish /banjoish things (melody) - often with two of the people who have contributed to this thread so far.

I make it a rule NEVER to play guitar if there is another guitarist playing because inevitably there will be a clash of chords as no two musicians will agree over what the chords are without arranging them in advance. There is a monthly session in our area with some fine musicians as regulars, but I have stopped going because there are three or four guitarists contributing regularly and the result is often a messy discordance.

I play in a six-piece dance band - again both chords and melody - and I always make sure when we introduce a new tune that what I am playing chords that agree with the inventive accordionist and the bass guitarist. When the accordionist can't make it, the dep. is a G/D melodeon player and then I have to play a much simpler range of chords simply because the melodeon - whilst being ideal for dancing - has a more restricted palette of chords.

Even although guitar was my first instrument, my only instrument for many years, I think that overall sessions are better without them.