Invite only help

hi i need help i would like to set up an invite only on my site im using phpbb3 and i know you can change the settings to admin to activate members but i know there are ones out there that when members join they get a certain amount of invite codes so they can invite peeps

anyone no please help

Such feature doesn't come with the standard installation of phpBB3, but I'm sure there will be some mods/hacks on the official site.

http://www.phpbb.com/mods/db/