I wasn't sure where else to ask this question and couldn't find an answer on the rental company website - I need to pick up a rental car about five days before my husband will be able to join us, but I need him to be an additional driver. Has anyone done this? I have a vague memory of the additional driver having to be present to be added, so I was wondering whether I could add him by going to the rental agency on the day he joins the trip.

Because different companies may have different policies, I'd maybe e-mail or use twitter or chat to ask the specific rental company. If that doesn't get the info you need, one option could be to stop at a rental location en route after meeting up with your husband and have them add him to the contract.

I do think you are going to need to visit the rental counter so he can present his driver's licence and credit card. But it isn't unusual to add a second driver part way.

This is from Costco's FAQ (by the way they waive a second driver fee).

https://www.costcotravel.com/Info/Rental-Cars-FAQs

Thank you!