add-user-to-a-particular-group-after-registration.php

<?php
**
 * Added to group automattic after succesfull user registration.
 *
 * @param string $user_id Get a user id.
 */
function wbcom_add_user_to_group_after_registration( $user_id ) {
	if ( ! function_exists( 'groups_join_group' ) ) {
		return;
	}
	/* Change the group id here. */
	$group_ids = array( 12 );
	foreach ( $group_ids as $group_id ) {
		groups_join_group( $group_id, $user_id );
	}
}
add_action( 'user_register', 'wbcom_add_user_to_group_after_registration' );