Adding a user through the Network Admin area shows a very limited range of options.
There are additional options when adding a user through one of the sub sites.
Adding a user through a single / stand alone site gives even more options.
There are a few WordPress Trac tickets that bring up similar concerns.
There is a Trac ticket that needs to be included before explorations can happen.
REST API: Improve users endpoint in multisite
“REST API users endpoint needs to be improved to support multisite behavior.”
A few mockups added to the Improve UX on Add New User screen trac ticket.
Another mockup with a multi select drop down.
Here is a wireframe for adding roles and users to sub sites through the all users screen.
It would be better to have a lookup system. Add user and one begins to write username or e-mail address and a suggestion of existing user shows up or a suggestion would show up to add a new user.