To answer your question, most groups I form have members who range in location from Fairfax to DC. I don't form city-specific groups yet. I just started the Reston group last week in hopes of helping out some of the moms who are even farther out West (Chantilly, Centreville...).
You can email me directly via our webpage: http://www.meetup.com/Mamistad/ if you have more questions.
I hope you're feeling great!