grouplet

Very nice. It seems identical in functionality to my own side project: https://grouplet.cc, however you have do have a nicer UI because I'm not the best designer in the world :-)

Grouplet is free, and has quite a number of groups using it. It's so cheap to run that I didn't feel it was worth charging for until I have an insane number of emails being sent every month.

Good luck, I hope it works for you!