A lot of very useful information in this thread,but the basic problem is - IS it a session or an Open mike/singaround.I have been to events advertised as sessions which most attendees used as a place to ego trip and play complicated arrangements in weird keys.Despite the organiser insisting it was an'Every body will join in'session.
IMHO a 'Session' is somewhere where everybody can join in and learn by playing with other musicians,so turning up at a session and NOT tuning in to everyone else is at the very least impolite.