1. Multi Authors/Contributors is already a feature of WordPress. You just need to let your visitors register for an account by creating a link anywhere on your site (preferably the header or sidebar) with one of the following two structures:
Use the User Role Editor to make changes to what each type of contributor can do.
2. Use Mingle Forum or BBpress. Both come with shortcodes you can insert into any page you want. Just make a page called "Forums" and put the shortcode in it.
Mingle Forum: http://wordpress.org/extend/plugins/mingle-forum/
3. Use s2Member Framework. You can actually use this for normal user roles too instead of User Role Editor.
4. Use Social Login and Social Sharing by JanRain.
As a general word of advice, I found all of these plugins just by searching for general keywords about the stuff you wanted.
You can use Google or the Plugins search form, which can be found at the top of this site as well as on your administrator bar (Plugins --> Add New).
Please make an attempt to find your own plugins before posting next time. I don't mind helping those who have a hard time finding stuff, but it would've taken you about 5 minutes to find all of those on your own.