A member+ is a member who made more than 24 posts. Members+ gain access to the After Midnight forum, where more mature discussions are held, and the Rock Circle, which is the GoF's parliament.
The ranks are connected with the postcount, and are divided as follows:
Newbie: less than 50 posts
Junior: 50-199 posts
Senior: 200-499 posts
Queen: 500-999 posts
King:1000-4999(?) posts
There are higher ranks such as Ultimate LBT fan and LBT-workaholic but I can't tell you how are they precisely divided (I forgot). There are special ranks reserved for the site staff, and those are Moderator, Global Moderator and Admin.