BuddyPress - Auto join group

I wanna automatically add new registered user to corresponding group by Buddypress.

Here is my code put in the theme's functions.

    function automatic_group_membership( $user_id ) {

       if( !$user_id ) return false;

        $join_group = xprofile_get_field_data(2, $user_id);  // 2 is xprofile field_id

        // conditionnal
        if ($join_group == "Group1") // Group Name
            $group_id = 3; // Group id

        // action
        groups_accept_invite( $user_id, $group_id );

    }

    add_action( 'bp_core_activated_user', 'automatic_group_membership' );

But it doesn't work.

How can I figure out this problem?

via Chebli Mohamed