@Adriano Faria
It would also be good to allow a member to claim a new members as a member that they invited to the forum in case that member signed up without entering the invitation code in instances where it is not required to sign up and remains optional. I think this needs some kind of validation to confirm they did invite them so it may be one of the two following things are in place;
Members email address is on their Invite List and they register without the code, it still shows as converted to the member that issued the invite.
In cases where it was word of mouth, the member that told them about the forum could claim that member was invited by them and it would then need the invitee to confirm that they were invited by this member.
You may even feel it appropriate for someone registering on the forum to be able to enter a member display name as the person that invited them in place of the invitation code.
Just a few suggestions to consider.