There is only four off us in our group. I'm mono, my girl only has sex with myself and her lover, his wife is the only one that goes outside of our group. She is very careful, and gets tested regular. She does not enjoy having outside relationships anywhere near as much as she used to though.
We don't really have rules to be honest, I don't think we need to. We do listen and respect each other though.
It would be very difficult for me to except my girls tummy swelling from her lovers seed. She knows this, and I think that is the closest thing we have to a rule. She must be very careful with her birth control!