Tomaz Kastrun has a script for us:
Determining and checking who has access to a particular workspace in your organisation can be time consuming task. You can always retrieve the list of workspaces and access the list of all users with PowerShell cmdlet Get-PowerBIWorkspace.
The results of this cmdlet gives you a great way to get to valuable information easy and fast. And the following script can do just that.
Click through for the script, which also excludes personal workspaces. Tomaz then provides a bonus script creating a graph of how users and workspaces connect.