How to make a private forum with an invitation-only registration?

[Omar]

http://www.lefora.com/
Just a few clicks to get the discussion started on Lefora free forum hosting service. With community Moderation and Anti-Spam support, email notifications, RSS, newsletters, and search engine (SEO) friendly, videos, photos, threaded discussions, unread tracking and rich text editors, and much more.

http://tal.ki/
Now you can embed a forum just like a YouTube video. Your forum will be integrated with Facebook, Twitter, Google, and other services so members can skip registration. Turn passive blog and content readers into participating members, contributing content.

http://www.vanillaforums.com/
Vanilla is a simple discussion forum that you can use to build community around your website, brand, or business. You'll have access to a fully functional forum, themes, categories, all of the admin features you need - Free.

http://www.yuku.com/
Yuku is about communities, discussion, profiles, images and blogs - all in one place. You can sign up to Yuku and create a free account. Each Yuku user account can create up to five Profiles. And each account can build message boards, a blog or create a gallery.

http://www.pearltrees.com/
Pearltrees lets you organize the web your way. Use pearltrees to keep at hand the contents you find everyday on the web, to discover new contents from people who share your interests, to drive through your own web... and contribute to the first human-powered organization of the web.