Adding an existing guest author when network user isn't added to subsite error

An `Invalid user parameter(s)` error is thrown when adding a guest author that exists on the multisite network but the user isn't a member of the current subsite. Autosuggest doesn't display the existing user, but trying to add a user with an existing name throws an error.

Steps to reproduce:

- On a multisite install with 2 subsites:
- Add a guest author called "John Guest"
- Change to the other subsite
- Try adding a guest author called "John Guest"
- Request shows an "Invalid user parameters" error