Bulk join user of certain role to BuddyPress group when created

bulk-group-user-join.php

<?php

add_action( 'groups_create_group', function( $group_id ) {

	// Get users of certain roles.
	$user_ids = get_users(
		array(
			'role'   => array( 'subscriber' ),
			'fields' => 'ID',
		)
	);

	if ( empty( $user_ids ) ) {
		return;
	}

	foreach ( $user_ids as $user_id ) {
		// Add them to group.
		groups_join_group( $group_id, $user_id );
	}
} );