@Huma0 - you can't. Even if they add the guest on the reservation, emails and messages only go the original booker. Now, I learned this when trying to resolve a similar issue as a co-host - now that they've eliminated co-hosts, this issue may have a different answer or the answer was only because I was the co-host, but I doubt it. At the time, after many emails, the response was that, even if the guest adds guests, even if the payment is split, emails from you are to the originating guest only.
They can, however, provide you with a private email of the extra guest and you can communicate with them in that way. After my experience, I would recommend emailing BOTH guests with the same email - one through the messaging system and one through the private email. My situation ended up with a whole lot of last minute negoitations because the guests were not on the same page with each other about their reservation.