Invalid principalSet member

[jamescrosswell]

I'm trying to grant GitHub Actions access to push containers to my artifact repository, per this blog post (which is the most comprehensive tutorial I've found on this so far). All seems to go well until I get to the "Allow to push container images" step at which point I receive an error:

INVALID_ARGUMENT: Invalid principalSet member

So, great... the argument I'm providing is invalid. Fair enough. How do I know what a valid principalSet member is though? What about the principalSet member that I'm providing makes it invalid? How can I debug this and work through it basically???