If you wait until the whole flight is sold out, you can't add the companion. But, as soon as you add the companion, all changes have to be made on the phone instead of online.
So, for me, I'm comfortable leaving the companion off for a while to potentially recapture points if price goes down. But, once WGA sells out 1) I worry about the whole flight selling out and 2) it's extraordinarily unlikely I'm going to see a points drop. That's when I add the companions.
There's no benefit difference in which seat a companion is assigned based on when they are added to the reservation. If you book WGA, WGA+, Anytime, add EBCI, or are A list as the primary ticket holder, your companion will be checked in sequentially with you.
If you book Business Select, your boarding position does *not* carry over to your companion, who will get whatever the next position is at their 24 hour check in. You can either 1) hope that there's still A 1-15 available to purchase once he's checked in or 2) book yourself one of the lower ticket levels such that you know you'll both board sequentially, even if it's later in the boarding order.