The system administrator needs to have access to all needed organisations. Please raise a support case if you do not have the right level of access.
To add an existing user to an organisation, go to users
And search for your user
Navigate to Organisation Access and click on the three dots
Complete the details and press OK
If the organisation has multiple portfolios, the user will need to be added to the relevant portfolios as well. Follow the same process, except add to the portflio list.