Let me speak to this from both sides, as an equal opportunity offender as well as in-crowd member.
Some folks, like me, when they first show up in a song or music circle, are nervous. And folks like me 1. make too much noise when we're uncomfortable, 2. are less likely to be on pitch and in time, and 3. don't listen worth a hill of beans.
So. You can post rules, and when you point them out to me, it may work, or I may just get my feelings hurt. It helps to bear in mind that a lot of people are at their worst when they first come into a new place. Just about EVERYONE is insecure in that situation. So you want a fast way to get past that.
What works on me, and I have applied to others with sucess is to be as kind as possible. Put out requests in positive and nonjudgemental ways, like "would you all join me on the chorus?" NOT "Don't sing on the verses!" Or, "Let me try this alone, I'm not sure I know it well enough to sing it with accomaniment."
Too noisy? Try making the thing quieter, and see if you can lead people down. It didnt work? Say before you start, " I want to try this one really quiet... (maybe add,) so I can hear Jim on the kalimba on the break."(or some such)
Anything that includes the Newbies in, praise, recognition, kindness, noticing what they do well instead of withdrawing and sitting with your friends. It is however also important to do this sincerely, otherwise people will just feel condescended to. So, in the immortal words of Thumper: "If you can't say anything nice, don't say anything at all." An awful lot of problem people turn out to be much better when they get comfortable and relaxed. Including me. So cut us some slack. As a last ditch effort, speak to me outside of the circle, alone, and ask me to change something that's driving you nuts then.
The other half of this, of course, is to show tolerance for the people who don't have any, and humor them when they sit there and snipe. Don't take it personally. Think what a hard life they must have living with themselves all the time and be grateful you only have to do it on first and third Saturday nights.
Blessings,
Barbara