Hi Terri,
If you decide to have stackable discount codes, the registrant will be able to add them at the very end of the registration process on the Payments page.
Once they enter a first discount code and submit the request, a brand new empty discount code box will appear that will allow for another one to be inserted there.
You can share discount codes within emails and this is one of the best ways to do so.
We have a great article on discount codes: Creating Discount Codes.
let us know should you have any further questions.
Best,
Federica