"I'm warning you, I do this one in VERY free time" - sometimes works, but not always.
If I'm hosting a session, I'll usually start by saying "Please say if you're happy for folk to join in, or if you'd rather be unaccompanied" - but even that doesn't allow for late-comers.
And a hand across the fret/finger-board of any stringed instrument has been seen to happen - and work!
And surely it's part of etiquette to LISTEN to the lead player/singer and follow them if you are permitted to join in, and not go off on your own tangential trajectory?!