Many new users wonder what hidden rooms are and how one can use them. The answer is really simple.
Hidden rooms are rooms that don't show up in the lounge. That means nobody will be able to find your room in the list of rooms. The only way others can join a hidden room is using the URL. Simply copy it from the URL bar and send it to the people you want to be able to join your hidden room. The people wanting to join a hidden room have to be logged in, then they simply have to paste the URL in their URL bar and hit enter.
In order to create a hidden room, you must tick the "Hidden Room (alpha)" box on room creation.
Since this feature is in an alpha-stage, it probably contains many bugs. One of the known bugs is that when you try to join a hidden room, it will glitch sometimes. In order to fix this issue you need to completely log out, log back in and try joining the hidden room again.