How to enable Guest Staff in MS Booking

Hi, 

I'm trying to add a Level 2 support engineer to my booking service as Guest (role). I'm able to add him and Bookings does send him an email saying that he has been added to the calendar and would get invites from the calendar. However, that email doesn't have a link for him to accept the invite. 

When I go to assign a booking to him as team member or Administrator, he does not show up in the list of staff to assign the booking. Nor can I type in his email address and add him as staff to the booking. When I look for him in the Staff settings, he is listed there with the red badge "Not bookable - pending for approval". 

How is he to approve an invite which doesn't have any links to approve! or how else am I supposed to get it done?