The "dive-in" policy is normal at music (instrumental) sessions. It's also pretty common at mixed song-and-tune sessions.

I think the use of the word "session" should give people a clue as to what they're in for. If, on the other hand, you say "singaround", it suggests that there will be some sort of turn-taking, and that's it's predominantly singing.

The other unwritten rule - or habit, if you like - is that at a "session", it's generally considered OK for musicians or other singers to join in with whoever starts a song or tune. There may be occasional solo "party-piece" tunes or songs that make this impossible, so people will just listen, but most of the time there will be some participation and improvisation from others in the room. At a "singaround", participation is usually confined to singing along in choruses.

Marje