American Atheists code of conduct for conferences.
We have been very fortunate in our freethinkers group to have never had any trouble but I have heard stories of a few groups that have. Maybe living in an ultra-conservative area we're a little more sensitive to where people might be and the experiences they've had in the past.
I think it's a good idea for any group to set a few rules or policies for how people will behave towards each other. The best time to do it is BEFORE an issue comes up but most of the time it ends up being discussed afterward.