// Send a custom email notification when a user joins a group
function send_custom_group_join_email($group_id, $user_id) {
    $user_info = get_userdata($user_id);
    $group_info = groups_get_group($group_id);
    wp_mail($user_info->user_email, 'Group Membership Confirmation', 'You have joined the group: ' . $group_info->name);
}
add_action('groups_member_joined_group', 'send_custom_group_join_email', 10, 2);