If you have external administrators and you want to give them SignatureSatori admin rights, you can create a Googl'e Workplace account and don't pay for a license.
This is called a Google Cloud Identity Free account.
Create the Cloud Identity Free account under Google Workspace
1. Create a new Organizational Unit in your Google Workspace
2. Turn off the auto-licensing in your Google Workspace
3. Create a new user without the Google Workspace license
(this user will automatically become Cloud Identity free account)
Now, you can find the newly created user and check if the Google Workspace license is truly not assigned.
Please take a look at these Google Workspace support articles for more details:
Load new users from Google Workspace
Start Synchronization with the first option User list - Add new users and remove deleted from G Suite selected.
Assign admin rights to this newly loaded Google Cloud Identity Free account in SignatureSatori
This account will look like a standard account in your SignatureSatori.
If you loaded it successfully and you can see it in your user list, you can assign admin rights under this link:
Check out what admin rights you can assign.