New User ID not working in custom function with user_register hook

I'm adding data to a table whenever a new user registers on the site.

add_action( 'user_register', 'auto_follow_admin', 10, 1 );

function auto_follow_admin() {
    global $wpdb;
    $wpdb->insert( 'wp_um_followers', array( 
    'time' => current_time( 'mysql' ),
    'user_id1' => 1, 
    'user_id2' => $user_id 
        ) 
    );
}

Everything works fine apart from the the new $user_id which isn't being passed to the query.

The stored data after it's run looks like this :

time : 2015-08-27 14:08:32
user_id1 : 1
user_id2 : 

Is global $wpdb; resetting the user_id?