Registering is not necessary to use the server, but it has some functions:
1. It protects your username, so only your certificate (that gets generated when you start mumble) can use this username.
2. It is necessary for defining permissions who can enter a room and who not, mainly used for the private zone.
3.It is necessary for becoming admin in a permanent room.
If you want to be registered go to the "I want to be registered"-room in the landing zone, or better write a quick message to an admin that you want to be registered, who the admins are is written in the welcome-message, or do both.
When you are registered you can also get your own permanent room, where you can be admin, for this go to the "I want a permanent room"-room or better write a quick message to an admin, or do both again, similar to the registration.
But only ask for a room, if you really need it. (for example don't ask for a room, if you just want to meet once, or at least tell then, that it can be deleted after).