Normally I would have preferred this to be the first reply, but it can't be helped.
The mod is an advanced version of the existing Invitation Only Mod. It was created for the phpbb.de mod contest (credit where credit is due) and implements many feature requests submitted for "invitation only".
Primarily this is the "User invite Users" part. (*cough* gmail *cough*)
This is what people trying to register will see:
The role choosing page for normal users. Notice the new "invite" option in the header.
The main admin config page:
The admin list:
The invite track page:
leads to (among others) to this:
Also, the admin page to create special codes (multi-use, custom code etc.)
I won't reply to installation problems, unless you have at least an installation attempt with easymod
to show. The easymod output will give a good hint on where to look for the problems; they will also be easier to reproduce. Last but not least: most likely there won't be any problems, thanks to easymod.
At the moment the mod is not compatible with the Categories Hierarchies mod, due to the changed group system in that mod.