J.J.
There are also a few groups on Yahoo. Look for Loudoun Mommies Graduates on Yahoo Groups for one that was recently created for moms in Loudoun with babies 4 months and up. I believe they will meet mostly on Saturdays to accommodate mothers who have gone back to work.
There is also a group that meets at Loudoun Hospital on Thursdays. The group is for moms with babies 4-8 months old. My baby is nearly 3 months so we're not there yet, but I attend the group for babies 0-4 months. It is not so much a playgroup but almost more of a support group. It is facilitated by a nurse and the mothers discuss milestones and concerns and get feedback and advice. I don't know what time the Thursday group meets, but I believe you should be able to find more info on their website.