If someone has come to your place, it's expected that they'll get back the same way they came. If you want to be really nice, and you live far out in the suburbs, you could offer to drive them to the closest metro stop--we have some friends who do that and it's very much appreciated. But you should not feel obligated to offer a ride that you don't really want to give.

If you and a bunch of friends are all meeting up at a location that isn't your place, and everyone's arriving or going home at the same time, it's kind (but again, not expected) to offer to pick someone up or drop off at their house, or a metro station, if it's a short detour for you. This is especially true for a potluck or other event where people will be bringing a lot of stuff; it's a pain to haul around on public transit.

You will likely get people reaching out when they need to move apartments... because let's face it, trying to do that without a car is a huge drag. It's always okay to refuse--just say you're busy that day-- but helping a friend move can do a lot to solidify the friendship.