An organization administrator can invite his/her team members to this organization but, not all of the users are able to login to our dashboard.
This problem usually occurs when the user enters with Sign Up in the dashboard instead of invitation link. This will let the invitee create another organization. Thus, the invitee can no longer join the intended organization as the organization is not shared due to security reason.
In that case, please ask the user to login to the account again then delete the account following this instruction. Then the user can be reinvited via dashboard members page.
If this was not successful, please let the administrator file a ticket to us. We will be happy to assist you move the account to the target organization.