Site admins should be allowed to add existing users without also enabling creation

[nacin]

Blog admins are allowed to add users to their multisite blog when the "add new users" network setting is turned on.

Unfortunately this allows them to both add existing users, and create new users. There should be a way, potentially just with a filter versus an option, to limit that to only add existing users, and prevent the creation of new users without going through the public registration process.

Use case: WordPress.com does exactly this.

Alternative use case: WordPress.org uses a pretty crazy SSO setup. We should not be allowed to create new users through the admin, but it would be very helpful to allow administrators to add users to a blog without going through the network admin (or asking a super admin).

Real world use case: Any blogging network like WordPress.com, or crazy synchronized login system like WordPress.org, would want this.

[jane]

I would actually argue the opposite, that the .com setup is aggravating, and the ability for an admin to set up new users at once in the admin rather than having to go through the invite process (and the confusing signup for blog/account split) would be a major improvement.

[jeremyfelt]

Does the show_network_site_users_add_new_form filter introduced in [16891] via #15793 account for this already?

[nacin]

This doesn't have to do with #15793, but this appears to be invalid.