Problem
You are getting the following error when inviting a user:
This might be because they already have an account on the platform. Each user can only have one account per email address, so if the person you are inviting already has an account associated with the same email address, they cannot create a new account.
Solution
Check the Users list for an existing account with the same email address as the one you attempted to invite.
If you are a Group Admin and cannot find an account in your group, it might be located in another group. Contact your platform's Company Admins for assistance in this matter.