When a user joins a BuddyPress group, this action adds that user to all ancestor groups.

bpgh-add-user-to-all-ancestors-on-join.php

<?php
/* groups_join_group is called whenever someone joins a group. */
add_action('groups_join_group', 'group_join_add_ancestors', 10, 2);
function group_join_add_ancestors( $group_id, $user_id ) {

	// I'm assuming that you'll want to join to all ancestors:
	$ancestor_ids = hgbp_get_ancestor_group_ids( $group_id );

	// To only include groups that the user can visit, use this version:
	// $ancestor_ids = hgbp_get_ancestor_group_ids( $group_id, $user_id );

	foreach( $ancestor_ids as $ancestor_id ) {
		groups_join_group( $ancestor_id, $user_id );
	}

	/*
	 * Note that this will do some recursing because groups_join_group()
	 * will fire the action "groups_join_group" for each group joined.
	 * If we only joined to the parent group and relied on the
	 * "groups_join_group" action to fire the next step, the method could
	 * break if the user already belongs to one of the parent groups.
	 */
}