Ok, I was able to invite the new users, so the urgency on this has gone down.
I think the problem is related to the “default plan” selector. It defaults to “professional” and I want “ultimate”. If I touch the default plan selector, the page breaks. If I leave it alone, I can add users.
Changing the default plan doesn’t seem to actually change what plan the users get. I need to set the plan for each user manually. They’re defaulting to professional when I want ultimate.