If the page will not respond, we'll have to perform the basic troubleshooting to fix this. Let's start with logging into your account through a private browser. It's the best spot to identify web issues. These are the shortcut keys:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Safari: press Command + Shift + N
If you can add a new user successfully using the private browsing session, go back to your regular browser and clear its cache. It helps avoid and fix web issues.
You can also use a different web browser to narrow down the issue.