@Anomie @Tgr So I wrote a proposed patch, which fixes the most important feature (the checkbox and error message when it's not checked), but I still have some issues with certain features -- can you take a look at the patch and see what I'm doing wrong and teach me how it should be done?

To reproduce the issue I described in the commit message of the aforementioned patch:

1. Check out NewSignupPage and SocialProfile with git clone (NSP uses the UserRelationship and UserStatsTrack classes provided by SocialProfile)...or alternatively you could just comment out the offending code in the NewSignupPage::onLocalUserCreated function and replace it with an error_log call or somesuch to see if that code is ever reached.
2. Load it in LocalSettings.php with wfLoadExtension( 'NewSignupPage' ); and set at least $wgForceNewSignupPageInitialization and at least either $wgAutoAddFriendOnInvite or $wgRegisterTrack to true
3. If you set $wgRegisterTrack = true;, you need to (manually, sadly) create the user_register_track database table: php maintenance/sql.php extensions/NewSignupPage/user_register_track.sql
4. Try creating a new user account via a URL like /index.php?title=Special:CreateAccount&referral=Foo&from=2 (where Foo is the name of a valid user account)

The idea here is that when a user signs up via such a link, them (the new account) and the referring user (Foo) are automatically made friends when $wgAutoAddFriendOnInvite is true. When $wgRegisterTrack is set to true, an entry gets written to the user_register_track table for each signup action taken via such a link. The new user account's ID and name are stored as ur_user_id and ur_user_name, respectively, whereas the referrer's name and ID are stored as ur_user_id_referral and ur_user_name_referral, and the from URL parameter's value (an int) is stored as ur_from.

Thanks in advance for any and all help you're able to provide me with this matter!