wpmem_pre_register_data

add_action( 'wpmem_pre_register_data', 'my_reg_hook' );
 
function my_reg_hook( $fields ) {
  
    // The following can be used to output what is in 
    // the $fields array so you can see what's in it,
    // and then it quits:
    // echo "<pre>"; print_r( $fields ); echo "</pre>";
    // exit();
  
    // In a real, working use of this action, you can
    // do whatever you need to do with registration
    // data before it is inserted.  
    
    // The following is an example using it to perform
    // validation, you can stop registration and return
    // an error message to the user by globalizing 
    // $wpmem_themsg and giving it your error message:
     
    global $wpmem_themsg;
    
    if ( $fields['my_field'] != 'some_criteria' ) {
        $wpmem_themsg = 'your error message...';
    }
    
    return;
}