Affiliates plugin allows you to set up an affiliate marketing program for your users tracking their referrals, clicks and sales.
E-Newsletter plugin adds a professional management system for your email newsletter and subscribers to your WordPress website.
Invite Plugin allows your users to send out invitations to friends to view or join your WordPress site.
The Private Messaging WordPress plugin from WPMUDEV allows your users to chat and share files safely on your website through direct messages.
Social Marketing plugin allows you to offer incentives for users sharing your products or services on Facebook, Twitter, Google+ or LinkedIn.