Yes, there is a difference.
Anyone can sign up for this forum. There are different areas of it that are available to different people, depending on whether or not you're a 501st Legion, Rebel Legion, Cloud City Garrison, or Kashyyyk Base member.
To become a member of Cloud City Garrison, you need to build/put together/buy and fit a costume to meet the CRLs (costume reference library), submit an application with pictures of you in your costume, and have it approved. Becoming a member of our garrison also makes you a member of the 501st Legion.
Please do get started building your costume! While you're working on it, you can check out the Spotters board here on the forum for opportunities to come out to some of our events, get a feel for how things work, and meet some other members.
You can also read through this for even more info about joining up.