Saving user meta “member_id” based on user role

Try this:

function assignuserid($user_id) {

    $user_meta = get_userdata($user_id); 

    $user_roles = $user_meta->roles;

    if (in_array("subscriber", $user_roles)){
        global $wpdb;
        $latestid=$wpdb->get_var("SELECT meta_value from $wpdb->usermeta where meta_key='member_id' order by meta_value DESC limit 1;");
        update_user_meta( $user_id, 'member_id', $latestid+1 );
    }
}
add_action( 'user_register', 'assignuserid');