User ID in Use
I tried to add a user to an API access group that is not in that access group yet, but when I got to the last step and clicked "send", I got the error message, "The userid is in use. Choose another email", which should not happen if we interpreted the instructions correctly.
So are we supposed to use two e-mail addresses for the same person? If that is the solution, it seems a little non-intuitive...
It's not really the same "person" though, it's a different application (API client app).
Your API-only users don't need to be emailable, so you can use <your-app-name-and-environment-here>@email.invalid as the address. (The choice of .invalid was not arbitrary, see https://nation.marketo.com/community/product_and_support/blog/2017/03/04/be-smart-about-dummy-email-....)
Second Sanford here. And in addition, the other reason could be that this person already have user in other Marketo instance. If this is the case, either you have to use a different email or enable the Universal ID on the Marketo instance. With Universal ID, you can access multiple Marketo instance with same login.