Basically for my project I want it so after a user has registered… they can invite their friends to the site too.
Something like Facebook has where you can log in to your live, gmail, yahoo whatever and invite your contacts on your e-mail to the site.
Anybody know how to go about doing this or know of any examples I can read about?
Thanks 
Two things to google for OAuth and OpenID, you might find a comparison of both that will point you in the right direction.
Do you know of any tutorials about what I want?
Try this and then [URL=“http://www.lmgtfy.com/?q=OpenID+tutorial+php”]this - if you find a good one then you could post the link back here.
why not write your own logic to handle it? I can provide more information. if you’re interested let me know
You can check this http://openinviter.com/
its free.