Invite a Friend

I invite the members to be logarem on my system, would have any option to do this, I have a system where members can join only with invitations from other members

Also interested in such feature:

I want to make INVITE ACCESS ONLY to my site.
If some user gets to site he has only 1 field:

invite code:
[register]

if he does’t have this code he cant login to site.

Also i want users have an option to invite friend by email.

any plugins?