From that screen shot it looks like members are still able to tick the box and send an email to members who are already registered.

well its not that server intensive if you are checking against 3 to 4 databases (like if you have joomla, phpbb, vbulletin and some other database and you wanna check against all those for registration status). But if you are checking against 4+ databases then it will put very lil load.
We are currently working on that thing, optimizing script for at least 10 databases. Also we are developing such a facility that will check remote databases(if you are running more than 2 websites and you wanna filter out imported members from all those databases). :)

regards

May I ask - is there a German version for it?

another question: does gmx.net also include gmx.at and gmx.de?

Sorry, currently we are providing only English version.
Although you can translate it to German if you want because there are less than 10 english statements need to be translated :)
And yes GMX does include GMX.at and GMX.de ;)

how to make the invites as first step to register so he can not begin registeration untill he invites his friend

also what about predifined message for the site so the user just click invite

The one which is attached here sends only predefined message set by site owner.. Users are not allowed to change or attach anything new to it.
About 1st step as invite, if you are really interested then i'll code 1 for you and post in this topic till tomorrow morning (IST). I will PM you when its ready. :)

Custom message attachment screenshot posted here is of 8.0
The mod always sends predefined message to selected contacts but it also attaches custom message written by user to the original message. And it is displayed independently.

also
if the registered member use invite , it should add refrrer for him so he can gain more users that they join by him and this give him points as i use vbexprence for points system that calculate points for regsitering users comes through refrrer link of registered member

Edit : Working on that rite now. I hope it will be ready by tomorrow. You will receive it automatically when we release updates. :)

Can I import emails from say an excel file rather than an email addy file?