This error occurs when the users you are trying to add to an organization exist in another organization on the same Acoustic Campaign pod.
For example, email@example.com is a user on Organization A, which is on Acoustic Campaign You want to add firstname.lastname@example.org to Organization B, which is also on Acoustic Campaign This is when the error displays.
To work around this error, you can create a user name that is slightly different such as email@example.com.
This works because the user name is the unique identifier for your account. Because you must use bob.smith2 to log in, the Acoustic Campaign then knows what organization you log in to.
If you had the same user name for all organizations, Acoustic Campaign would not know what organization (or even user) you were trying to access.