bp_get_descendent_groups( int|bool $group_id = false, int|bool $user_id = false, string $context = 'normal' )

function bp_get_descendent_groups( $group_id = false, $user_id = false, $context = 'normal' ) {
	/*
	 * Passing a group id of 0 would find all top-level groups, which could be
	 * intentional. We only try to find the current group when the $group_id is false.
	 */
	if ( false === $group_id ) {
		$group_id = bp_get_current_group_id();
		if ( ! $group_id ) {
			// If we can't resolve the group_id, don't proceed with a zero value.
			return array();
		}
	}

	// Prepare the return set.
	$groups = array();
	// If a user ID has been specified, we filter hidden groups accordingly.
	$filter = ( false !== $user_id && ! bp_user_can( $user_id, 'bp_moderate' ) );

	// Start from the group specified.
	$parents = array( $group_id );
	$descendants = array();

	// We work down the tree until no new children are found.
	//while ( $parents ) {
		// Fetch all child groups.
		$child_args = array(
			'parent_id'   => $parents,
			'show_hidden' => true,
			'per_page'    => false,
			'page'        => false,
		);
		$children = groups_get_groups( $child_args );

		// Reset parents array to rebuild for next round.
		$parents = array();
		foreach ( $children['groups'] as $group ) {
			if ( $filter ) {
				if ( bp_include_group_by_context( $group, $user_id, $context ) ) {
					$groups[] = $group;
					$parents[] = $group->id;
				}
			} else {
				$groups[] = $group;
				$parents[] = $group->id;
			}
		}
	//}

	return $groups;
}