Invite Anyone
http://wordpress.org/extend/plugins/invite-anyone/
Invite Anyone has two components:
1) The ability to invite members to your site by email. The plugin creates a tab on each member’s Profile page called “Send Invites”, which contains a form where users can invite outsiders to join the site. There is a field for a custom message. Also, inviters can optionally select any number of their groups, and when the invitee accepts the invitation he or she automatically receive invitations to join those groups.
The email invitation part of the plugin is customizable by the BP administrator, via Dashboard > BuddyPress > Invite Anyone.
2) By default, BuddyPress only allows group admins to invite their friends to groups. In some communities, you might want members to be able to invite non-friends to groups as well. This plugin allows you to do so, by populating the invitation checklist with the entire membership of the site, rather than just a friend list.
Because member lists can get very long and hard to navigate, this plugin adds a autosuggest search box to the Send Invites screen – the same one that appears on the Compose Message screen – which allows inviters to navigate directly to the members they want to invite.
Invite Anyone features optional integration with CloudSponge http://cloudsponge.com, a premium address book service, that allows your users to invite their friends to the site in a way that’s easy and fun. Enable it at Dashboard > BuddyPress > Invite Anyone.