It's a great idea to make the invite friends option apart of the last step on the join form. So once they register it'll go to invite page, with an option that says "skip this step" and then go directly to the home page. **How about making an option in the admin page for that last step on the invite link to go to any page that we select** Thats good because sometimes you want to run site promotions and send them to that page first.
Addition. Along with inviting the friends, you should make it so that members can check to see if their friends are already registered on the site. So when it pulls the e-mails from their e-mail account it will compare that to the member database, and if their friends are already on the site give them the option to add those friends to their friends list. (Just like Myspace does) Great Addon

. It's also important that members have the option to select the friends that they don't want to send the invite to from their e-mail contact list.