In XMPP, chat rooms are called MUCs (Multi User Chats). MUCs must be entered explicitely. Most XMPP clients offer storing bookmarks for MUCs and then enter them automatically upon startup / login.

There are a number of existing MUCs on our server, many XMPP clients provide a way to display the list of existing MUCs and then enter a selecte one.

Just as well, each user can simply open a new MUC and manage it himself!
This happens automatically when you try to join an MUC that does not exist yet.

The JIDs of all MUCs on this server follow the principle "<roomname>@", a MUC where everyone is welcome would be e.g. "".

The roomname can be chosen freely under certain restrictions, permitted characters are A-Z, a-z, 0-9, dot, comma, underscore and hyphen. The room name can be up to 256 characters long.

Have fun chatting!