A couple people have been having problems with their invitations from other people, so I've made this short tutorial on how to use them.
1: Once you get your invite from another person, look at the message. There should be a long URL there.
2: Go to that URL, and fill in your first and last name, the account name and password you want to log in with, and your email address. Once you press Create Account, it's all set up! If you get a message saying that the invite has already been used, then you might be using Firefox, which has some problems with the invite page. Switch over to Internet Explorer for this page, and it should work.
3: Open up the UU client and, in the window where you select the shard (which will of course be D'mala), enter in the Account name and Password you just created, and click Login. Now you can create a character and start playing.
NOTE: Ignore the "Invitation" area when you create a new character. Putting an invite in that section was an old thing from Live, where people would get one-use passes into Live.