If you are here because WordPress just throw a “Please provide a valid username” error while installing WordPress 3.7+, you are on the right direction. Here what happen with the new release.
Using a single Users table for multiple WordPress sites is partially broken for WP 3.7+.
There is no perm solution for this issue yet, but there is a ugly workaround posted at wp forum. You can read it all here.
C'est la vie 