In the end, here is what we learned:
1) Alamo Insiders are supposed to receive an additional driver at no additional cost.
2) Sometimes this may not be the case based on coupons used.
I want to bump this thread, as it just saved me $100, or at least an administrative headache haggling at the rental counter over additional driver fees.
Last week, I signed up as an Alamo Insider. I then made an Alamo reservation using the VISA cardholders contract id [ 7015504 ] and $30 off coupon [ AD2338FDF ] linked from Mousesavers.com
I was happy with my rate, but curious as to why I was being charged $9.99 / day to add my spouse as an additional driver, when it was supposed to be free for Insiders. My res clearly showed my name and Insiders ID.

After reading this thread, I experimented and rebooked my res with the Insiders Contract ID [ 7015135 ] and the same coupon. I ended up getting the same exact rate as before, but now the res clearly reads:
Contract I.D. Alamo Insiders Members
Guaranteed Base Rate Included
(1) Additional Driver Included
And I was able to use the "SaveTime" feature to add my spouse for free. As others have mentioned, it shows $9.99/Day, but doesn't actually add any cost to the bill.
Again, thanks for those contributing to this thread, especially gmeh1. I'm sure this is a relatively common scenario for those trying to rent through Alamo.
Now, if you don't mind, I have a question about Alamo coupon codes.
The VISA Discounts link I mentioned above included a $30 off a weekly rental coupon code. I have since discovered codes for $35 and $40 off from other sources, such as BJ's Wholesale Club, where I am NOT a member. Are the coupons strictly tied to the group to which they were originally advertised? Is it risky or poor form to try other coupons?
Thanks