BuddyPress.org

I took a very quick look at the OpenInviter website. They state that it is Open Source yet they do not provide any link to their licensing information.

Also, there is no code repository, it is not clear exactly what the code does, and you have to provide personal information to get the code. This does not indicate open source to me.

The site offers some sort of a WordPress Plugin, but I doubt that it is released under GPL which is a violation of the WordPress GPL license.

So, you are on your own if you want to use it, but I’d recommend steering clear of OpenInviter.