In TG, the Organisation a User is created in is referred to as their "Home Organisation"

In order to access and view other Organisations, User accounts must be mapped to these Orgs. 

This can be done from within the Organisation. Go to Admin -> Users and Map Existing User

Alternatively, users with the Manage Support User permission on their account can set up other Users as Support Users

Any account that exists within the Partner Home Organisation can be given the Support User status. This will automatically map them to all Organisations under this partner 'invisibly'. 

Invisble mapping means that they will not show up in the list of users for an organisation.

Under Edit User:

This enables the user account to support all of a partners Assets/Organisations.