If you are trying to add someone to your account and getting a "Cannot invite user, already exists or been invited" error message (as seen below), it is most likely the person has already created an account independently. Please open a support chat or submit a support request and provide us the email address of the primary/parent account as well as the email address for the existing account you are trying to add in. We will be able to add the existing account as an invited team member.
We are working on an enhancement so you and the person you're trying to invite can resolve this issue yourselves. However, until we complete and release the project our support team can manually add the person to your account.