"Chatty Cathy" might be feeling like she needs to talk about herself so someone in your group will feel they have something in common with her and want to be her friend. I can't help but think to myself, how would I feel if I were in her place? If I were talking so much others wanted me to leave the group, I would wish someone would at least let me know what I was doing to offend the others. I would hope someone in the group saw something worthwhile in me to look past my faults, (...we all have faults). I would be devastated if the group started meeting at another location in which I had not been invited!!! You could start the group time in a new way more structured, plan to go around the room giving everyone a chance to say something about...something like the most memorable discussions they can remember in the group, and during that time mention how much you like it when the group is mellow.