Does the co-author already have an Adobe ID setup with the email address you tried to share the form with? If so, they'll need to log in to FormsCentral with that username and password.
I assumed this issue and simply went to the ”forgot password“ and the email never arrives?
Carl R. McElroy
1-888-278-1701 | MC 312-386-0244 | MM 952-278-1731
Could you PM me the email address of the co-author? I can take a look at the Adobe ID.