I totally get your frustration. I have checked into several mom's groups since moving to Cape Coral in July. There were a few who just didn't respond to me at all, and then others that said that they have firm boundaries and I am just outside of them. When I had my daughter 2 years ago, I was completely alone without any family and in desperate need of companionship. We lived in Tampa at the time and luckily I found a great mom's group. I ended up being involved in the leadership of the group and I am familiar with the politics involved.
We really didn't have boundaries as far as who could join, but it was really easy to get kicked out and after 40 members, they wouldn't let anyone else join. This caused a lot of hurt feelings for moms. The leadership was determined to do things their way.
Anyway, I would just tell them that there are other moms out there who really need them and try to explain why you feel the way you do. If they continue to reject the members, encourage them to tell the moms they are rejecting where they can find another group.
Because of the difficulty of getting into a group in Cape Coral, I just started my own. I started it a week ago and we already have 9 members. I agree that it should be up to the mom to determine how far she is willing to drive for a playdate. There are a lot of lonely moms out there. Hopefully as the need for groups arise, more and more groups will form to meet the need.
I hope all goes well!
T.