Whoa. mac_gyver was right, I have no clue how that happened. Thank you so much, I never would have even known to look for that.
After replacing show_join.php with a brand new show_join.php (with the correct line) I'm still getting this error when I try to join:
Deprecated: Function ereg() is deprecated in /home/fatedus/fatedus/loved/admin/show_join.php on line 141
And yeah, of course, this is the new line 141:
if( $_POST['email'] && ereg( $matchstring, $_POST['email'] ) )
Apparently the join form works 'cause I just got the confirmation e-mail that I joined.. I'm just also getting that error above on the confirmation page right after I submit the form.