To make a direct call to someone, just click one of the side window with a "blue head" on the right, and a "Make Video Call" window pops up, then click on the ">" and a list of names appears. Click on any nick to place the call.
If you did not join a room, then the list would contain the names of people who also did not join a room. This provides a way for people who do not know each other to make contact.
If you joined a room, then only people who are in that room are listed. This allows you to avoid being seen/called by strangers.