When adding users to a site, if you choose to select their roles separately:
You are shown a page where the pop-up menus are marked with asteriks to let you know that they're required. If you do not make a selection on these required boxes and click "Continue," the page refreshes, and you're back where you started.
There is no error message to tell you what went wrong
If you selected some roles but not all, your selections are lost.