The crowds would be much larger if most of us heard about this earlier than 11:58 am today. Absolute garbage organization for this one. I've marched in D.C. on more than one occasion and knew about the event at least a month in advance every time.
This is a response to the rapid escalation of events since the inauguration which only occurred just over two weeks ago. Things are moving fast. The past week has been dizzying, by design.
There will likely be a lot of protests by many organizations that may be organized at the national level or locally until things change for the better, people give up, or people are stopped by the forces we need to protest against. Try to find protests and participate in the ones that you can.
edit: I'm not going to tell you what groups to organize with, but look into groups that organize locally and nationally.