I'm not sure about Harmony Ridge - they do have a junior show team, and I had heard they may start an adult team - this said, I don't know about adult lessons.
Willowbend in Upper Marlboro has lessons - and they are great people. Also, Tina Emmerick at Oak Crest farm in Lothian has adult lessons and a whole bunch of super wonderful equine ladies (I used to ride on their adult team!
). Her lessons are all group lessons - but very reasonably priced at $25 for a half an hour. Lots of fun - and great way to meet new people in a very social barn.
If you were more interested in private lessons - my barn, Lucky Cricket in Huntingtown offers balanced seat equitation lessons. Very good, personalized lessons. They run $30 for 30 minute private.
Hope this helps!