I've been running a weekly session for nigh on twenty years now; no rules, per se, but rather consensual guidelines, to wit: electronics should be limited to tuners. We had a plastic electronic saxophone there for a while which diminished the overall listening/playing experience markedly. No instrument should be so large as to replace the space which could be filled by another person. We do have a small corner into which we can tuck a standup bass and player. There was once an incident with a cretin who, after permission was denied, still brought in his snare drum. While I am sure that I was more offended than he, he never came back.

On the other hand, the thread title refers to "song circles," which in my mind precludes the instrumental aspect. We also have monthly shanty/forebitter sessions. Instruments there, again by consensus, are limited to intros and accompaniments for forebitters and fo'c's'le ballads, not for shanties.

Just my own experience -- Tom