WordPress Social Invitations aka WSI is a plugin unique of its kind. After creating the apps for the desired providers, users will be able to connect, choose their friends and send invitations to join your site.
It’s built using open source Hybrid Auth library and currently supports 8 providers. It can be integrated with BuddyPress or the Invite Anyone plugin and also be used as a content locker. It works with shortcodes, PHP functions or by adding a sidebar widget.
The plugin is fully customizable and every template can be modified by following some simple steps. It works with the server email system or can be used with third party services such as Gmail, Mandrillapp, etc.