Does the registration include accommodation?
If not, can any locals recommend something cheap nearby? I was thinking about staying at the HI in Washington for an extension of the trip to see the sights, but that might be far to go each day, or is it?
No, registration does not include accommodation.
The conference site is one block from the new McLean Silver Line Metro station. I am not familiar with the HI, but if this is it, it appears to be 3 blocks from the Metro Center Metro station. Google tells me that the trip would be about 40 minutes by Metro.