As Trass already mentioned, yes, you have to sign your character to the guild on Raidar. It is not done automaticaly, because anyone could claim your character and infiltrate your guild’s forum and raid plan on Raidar. That’s why Raidar requires your raid leader’s or officer’s confirmation.

There are also other, much simpler ways to build a guild on Raidar. Guild leader can send e-mail invitations or generate special link that can be posted in your guild’s forum, etc. Coming to Raidar using these links will grant your characters automatic membership.

Anyway, when you create a new character, Raidar should give you some options about what to do, based on data from the Armory. If the guild doesn’t exist on Raidar yet, it will offer you to start it, if it exists, it will provide you with one-click sign-up link.