Bridget,
My suggestion is to go out to dinner before meeting. That way, all the girls feeling excluded can go out before meeting and have fun and maybe they won't feel bad when the others want to go out after meeting.
Also, try to have sisterhood events where people have to work in groups. Have the groups pre-assigned so that you can split up the "cliques" and get people to socialize that normally wouldn't.
You can even work with your personal development chair (if you have one) and have her do a program where you have to spend time with a different sister every week. Then people would turn in cards to the PD chair saying: "I spent time with _____," "we _____ (went shopping, talked on the phone, played video games, etc)," "something new I learned about her is ______."
Hope things get better.
