Aw, my comment got cut off. Here it is again:
I believe that those buses only accept passengers one way – into DC in the am and out of DC in the pm. There used to be one that also ran in the reverse direction (#921) but it seems to have been discontinued. Probably because there were only 1-2 other passengers the couple of times I rode it. It left from the New Carrollton metro and was very expensive (esp. adding in the metro fare just to get to New Carollton), had weird hours, was rarely on time, and the whole trip took 2x+ times as long as driving. Plus you still had to get to your office once you got to Annapolis, which is not easy.