cynthiablue, hm, the invitation system isn't extremely useful, as is.
One implementation I saw was for a local church. They didn't want to moderate who could, or couldn't join their website, but they didn't want random people joining either. So, they published an invite code in their newsletter and effectively locked people out from the joining the site that weren't in the know.
Another interesting implementation was used by a school teacher, she set up different invite codes for different classes she taught so she could keep track of where different users came from. Again, she required a code to register.
Regardless, the invite codes don't really promote your site, they control who gets in, or at the very least, how they got there, if used properly.
If there's something specific I can help you with, let me know.