I found the user-manager contrib, but it doesnt sound like it does what im looking for. I want to set this up kinda like a free e-mail service that anyone with an invitation code can use. Kind of like gmail, but i'll be the only one with the ability to send an invite. I could use the user-manager contrib, and just make the user id and password be like the invite code, but then i would have to change the id and password after they create an account to prevent them from creating additional accounts. At that point, i might as well just create the account for them. I know that the contrib allows you to set the max number of accounts that can be created, but then i would still have to change the info if i were inviting someone else.
So anyway, I'm really looking for more of a DIY type solution. I'm fairly confident that I can handle the progamming part, as I said before, I just dont know what files need to be modified.