REST API: User with `promote_users` cap can't add existing users to a site

[ocean90]

Adding an existing user to a site should be done by performing a PUT request to the user resource on the other site, see ticket:38526:7. This works if the current user can edit users.

But core also allows users with the promote_users capability to add existing users:

• https://core.trac.wordpress.org/browser/trunk/src/wp-admin/menu.php?rev=38827&marks=225,235#L219
• https://core.trac.wordpress.org/browser/trunk/src/wp-admin/user-new.php?rev=39006&marks=176,279-282#L278

[rmccue]

I'd be tempted to leave this until 4.8 and take a holistic look at multisite behaviour across all resources to make sure it works well. (We fixed #38526 in 4.7 since it was a bug, not to add full multisite functionality.)

@jeremyfelt Is this important enough to pull into 4.7?

[jeremyfelt]

Taking a look at this now for 4.7.

[jeremyfelt]

I started poking at this, but it seems like something that should be thought through some more first. Let's leave it at edit_users for now and then follow up in early 4.8.

[larsonreever]

take a holistic look at multisite behaviour across all resources to make sure it works well.

[jeremyfelt]

The original functionality for adding existing users was disabled in 4.7.3 via [40106] as it was not part of a clear strategy. Let's close this ticket in favor of #39544, which aims to improve the users endpoint as a whole.