When you create a user in the TG system that user account is created in the Organisation that you are currently logged in to.
So make sure that you are logged in to the correct Organisation when you create a User account.
This Organisation becomes that User's "HOME" Organisation - and their user account can only be edited in that Organisation.
A User Account can be mapped to other Organisations.
You should do this by going to the User's home Organisation and editing the user, and adding a mapping to the Organisations that you want that user to also be able to login to.
If a user is mapped to more than one Organisation they will be presented with a list of Organisations to select from when they login.