[miz]

you can do like that,

every user got invite slots. and keycode

now adding to registen form a value of code
which user registen need to enter

now when guest came to registen form and enter a code
its search in user who have the same code as enterd and then from same user
its take out 1 slot.
also i can make for you a system that puts random code to each user every day
so if user want to invite some1 he enter to his profile to see the code and give it to his invited user.

how is that sounds to you ?