I'm in Calvert Co also and ride St. Mary's and Rosaryville often. I actually just registered for the DCXC race at Rosaryville in June and the Wednesdays at Wakefield race series in June/July. I work as a tax advisor for this
company. That's not saying much though, I'll be beginner class hopefully not coming in last. Anyway, I usually ride solo, no reason other than I usually end up being busy when others are riding. I know the bike shop in Solomons would have more info for group riding. They are a great group of people at that shop, always happy to help with bike issues, sometimes on the spot and the prices for their work is very reasonable.
If you strike out there, check out the Rosaryville MTB group on Facebook, I see they normally have 9am Sunday rides (when I'm at church of course) and some other short notice rides.
Finally, I've seen posts on Facebook that there is a group currently building a public MTB course around Huntingtown in Calvert Co. Seems like a great opportunity to get out, help the sport and meet some people. I plan on helping out with the group when I see the next date to help build.
Good luck! I usually ride one day a week after work on my way home (DC commuter) and one day during the weekend, lemme know if you wanna meet up.