You can do this by going to /rift (say it in chat) then say /register <password>(replace password with a new one no one else knows). If it responds with:
Registered successfully. Now /login.
run /login <password> (the password from earlier).
otherwise if it responds with:
You will be allowed to register in …
then wait for that amount of time before you can register.
Info available here: Frequently Asked Questions