It'll get even easier on Thursday, check out https://github.com/arthyn/port for a preview.
What do people commonly discuss on it?
On that group, they mostly discuss urbit.
There are a lot of other interest groups with varying levels of activity: cat pictures, art, cryptocurrency, tech, emacs, politics, finance, ham radio, weightlifting, cars, tea... All the different kinds of stuff that people discuss on the web.
Once you have it running, go to http://localhost:80 to use the web interface (Landscape). The Urbit Community group is the most active. Just hit "Join Group" the main page and enter "~bitbet-bolbel/urbit-community"