WPMU DEV INVITE Plugin
Give your users a way to invite their friends and colleagues to check out their blog and sign up for your network.
WPMU DEV INVITE Plugin lets you send an invitation to friends and colleagues to view or join your site.
A marketing machine
Give real fans of your site a way to share it with others. Make your user base a powerful marketing engine.
Invite offers a strong code foundation making it easy for developers to integrate with others systems. Build a contacts importer for web-based email services like Hotmail, Yahoo or Gmail – and use Invite to email every contact in your address book.
Edublogs tested and approved
Prompt your users to invite people they know. Send invitations with ease using the simple Invite email form which includes the option to send a personal message along with the invitation.
Please visit official plugin page for more information