From LeakyPedia

The Three Broomsticks - Penn State University

Mischief Managed!- Chicago Marauders

The Group That Shall Not Be Named - Harry Potter NYC

HP-Ohio

Potterdelphia

HP-Orlando

S.L.A.W. - St. Louis Area Wizards

HP-MA

The Seattle Harry Potter Meetup Group

Los Angeles Dumbledore's Army Meetup Group

Rowling House: Greater Phoenix Chapter Bold text

Don't know if an online group counts, but I'm an administrator on the Babycenter Harry Potter board.

The Poll Castle Commentators and we meet at the comments section of the Poll at TLC. We are about fifty people in number who have been chatting for over six years now.