Sports leagues are good. DC Rugby lets people practice with no experience and even if you don't want to play going to their matches is a good way to make friends. They were advertising on the sub today so they shouldn't be hard to find. There's a ton of other intramural leagues as well.

Grad school is also still college so you'll still kind of have forced relations. A lot of feds are still hybrid/remote so DC can be a difficult place to meet new people. I would look towards your university first. If you need a friend, you can always DM for a drink.