I am the administrator on a brand new guild site. I have set up the ranks and all that fun stuff. However after people register on the site and I grant them access they still can not post on the forums. Not even the person I gave the rank of Guild Master. Any help would be great. Thanks!

Check the thread tag permissions in the Admin > Forum > Thread Tags (Categories). They're likely set too high.

The problem actually happens to be (in this case) that the accounts themselves weren't being promoted to the proper ranks, only the characters.