A lot of fascinating information here – my thanks to all contributors. Here are a few further thoughts from a latecomer with some experience of playing guitar in sessions.

Firstly it's worth remembering that you don't have to play all the time, and there are often good reasons for not doing so. If someone starts a tune you don't know, and the opening phrase tells you that it doesn't fit into a familiar pattern, sit back and enjoy it.   If you want to learn the tune, lay the pick aside and play along with bare fingers – in an averagely loud session, nobody but you will hear the mistakes.

Secondly, even when the tune is a familiar one, you must be prepared to adapt if the melody players are interpreting it in an unfamiliar way.   (This may mean shutting up entirely, or playing very quietly until you're in synch with their interpretation.)

Thirdly, you don't have to play every beat – and on very fast tunes it's usually better not to. Thrashing away at 6 or 9 beats to the bar on fast jigs or slip-jigs can produce a wall of noise that sounds unappealingly mushy – however fulfilling it may be as an athletic exercise for the player. Whereas a few rhythmic punctuation marks at appropriate points can really boot the tune along – and picking the right points to insert them is a real challenge for guitarists. (Especially during jigs that require no chords but Ami and G.)

Fourthly, you don't have to hit every string on the instrument every time. There are occasions when a two- or three-note stab makes a more incisive comment than a six-string chord. A well-chosen bass line also enhances many tunes – or why not try playing the melody? (If it's good enough for Doc Watson …)

Finally, on Greg's point that one guitarist per session is enough, and more only get in the way, I beg to disagree. IMHO it depends on how resourceful (and how tasteful) the guitarists are – and on how much attention they pay to each other and to the melody instruments.

Agreed, if one guitarist sticks to beating out first-position chords in a steady rhythm, then duplicating that will add nothing except extra volume to the mix. But a second (or third) guitarist can still enrich the sonic texture by using different chord shapes further up the neck – or by developing an interesting bass line - or by inserting a few tasteful rhythmic accents on appropriate beats. (If it was good enough for Eddie Lang and Lonnie Johnson …)

Wassail!