If you look on the website you will see that this option is available for the first 3 home matches from tomorrow. It's a long time since I've used it so I think the guest has to be in your friends and family list but not absolutely sure what conditions are. Hope this helps.
Previously the guest didn't even need a fan number as you don't assign the ticket to them. At checkout the ticket is assigned to the STH as they are the one eligible to buy the ticket.