I've found it relatively easy to add my membership number to bookings made using a corporate travel agent (Clarity Travel Management, in my case). It's just a matter of going to "Stays and Events" on the IHG website, scrolling down to "Don't See Your Stay?" and looking it up with the IHG confirmation number (this comes to me in the email from Clarity, under "Hotel Ref") and your name. Once the reservation has been loaded, there should be a big banner that says something about your number not being associated with the booking. If you click on this, it should then ask you whether you want to associate your number with the booking. It may then giving a warning about mismatched details, saying that associating your number with the booking will overwrite whatever is in it with the personal details from your account. If you dismiss this warning, it should then associate the booking with your membership number and show up with all your other bookings in the "Stays and Events" list.

I believe that an "authorized travel agent" is, basically, any TA that books through a GDS. I've found this to be the case with other chains, anyway (non-GDS bookings tend not to qualify, GDS bookings always seem to)