Point taken, Jack - I'm not familiar with this sort of session. In the sessions I go to, there are only a few songs and they're generally either a capella or accompanied mainly/only by the singer, possibly with a friend or partner. The other musicians tend to sit back and listen, or ignore it and use the break go to the bar. There are a few well known songs that other instrumentalists will join in, but that's the exception rather than the rule.

Marje