This can only be done through the administrator tool by following the below steps:

1. Launch the Privia Administrator Tool
2. Enter your credentials
3. Expand the Organization Node in the TreeView on the Right side of the Tool
4. Right click on that organization in the TreeView where the Workspace is contained in
5. Select "Show Content" from the context menu that displays, 
6. A list of all the workspaces in that organization should display
7. Select a workspace in the left-hand pane
8. Click the "Membership" button
9. On the new dialog that opens, the left hand pane will contain all available users in the organization and the right hand pane will contain all of the users in the specific workspace.
10. Add users to the workspace by selecting the user in the right hand pane and click the right facing arrow to add that user to the workspace

Notes:

Performing the above actions enables an administrator to add users to workspaces without having to actually be logged in to the Privia clients and selecting each workspace individually