Hey Hans,
Sorry for the delay in getting back to you - Spend a few hours with our team to make sure I had the right answer to this one.
Unfortunately, jla is right, there isn’t currently an endpoint that you can hit to return userId’s. I think the proposed solution is a decent solution for the short-term and I made a feature request internally to add an endpoint to return this data.
I will keep this thread updated as that feature request progresses. Thanks for the fantastic feedback, Community is exactly the right place to get your voice heard and features added to the platform!
Pete