If an employee has workforce set up and I need to add her as a user, how do I work around Qbooks not letting her log in as user because username is already in use?

Thank you for posting your concern here in the Community, dbasslaw. 

With the regard to the error received by your employee, it occurs when the email address has already been used. I suggest using a different email address for the said user to prevent getting the error.

You may also want to read this article to help you manage your users: Add and manage users in QuickBooks Online. 

I will always be around if you need more help in managing users. Have a wonderful day!