The short version is that Xanthus's mentor was a member of the Order, and through him Xanthus gained a connection to the Order which he is now taking full advantage of. His goal is to turn the Order into a partially humanitarian organization, and to see if it could potentially become an ally of the Rebellion. The full story can be seen in my application to join the indie faction (it's too many characters to post it here). Link
Baling said that members will have been members for at least the last 20 years.
ICly he originally joined as a part of a mission when he was a Padawan. Joining now to help the Galaxy and to try and keep the organization from becoming corrupt.
It's all explained in my app to join it on the sign up page
I hope those details weren't "and she's a rebel traitor" O.O might make life quite possibly shorter for poor Ral. Haha, but in any case I look forward to seeing how this plays out one way or the other.