BuddyPress.org

You need to provide the $type_id for the Member type:

$get_user_ids = $wpdb->get_col( "SELECT u.ID FROM {$wpdb->users} u INNER JOIN {$wpdb->prefix}term_relationships r ON u.ID = r.object_id WHERE u.user_status = 0 AND r.term_taxonomy_id = " . $type_id );

If getting your hands dirty with SQL scares you (like it does me) BP has its own functions:

$member_type = "doctor"; // slug of member type
if ( bp_has_members( array ( 'type' => 'alphabetical', 'per_page' => 1000, 'member_type' => $member_type ) ) ) {
	while ( bp_members() ) : bp_the_member();
		// code
	endwhile;
}