Missing option in Account / Users / Add
Comparing the "Add New User" form between old and new UI:
The form in the old UI has a button toggle between a "Basic" and "Advanced" form, which appears to do nothing but add an "Auxiliary groups" option to the form. I'm not seeing a place in the new UI form for either the Basic/Advanced toggle or the "Auxiliary groups" option. I personally don't think the button is necessary, but adding the "Auxiliary groups" field under the "Primary Group" option seems like a good fit and keeps feature parity with the old UI.