Robert Kampas

By default WordPress does not allow you to create users with custom password on multisite website. This plugin will add password fields to new user form on individual multisite website so that you could type in new user’s password. However, please be warned that it is not advised by WordPress due to possibility of registering users to network they should not belong to and possibity of resetting existing user’s password.

Multisite New User Form

This plugin allows you to create users with custom password on multisite website.

Pigrecolab

This plugin prevents the creation of new administrators, either by code, or by acting directly on the database. Ideal for locking possible backdoor hidden in code. 
BE CAREFULLY: DEACTIVATING THE PLUGIN YOU DON’T REMOVE THE BLOCK. YOU MUST USE THE PASSWORD IN SETTINGS / BLOCK NEW ADMIN AND DEACTIVATE THE BLOCK BEFORE DEACTIVATING THE PLUGIN
It provides:
<ol>
<li>A setting item to create a password to block o unblock a new administrator creation</li>
</ol>

Block New Admin

Works as advertised!!! Must be included in the core, sadly the core devs don't care or are clueless about security.

Excellent idea

A security plugin to block the creation of new administrators

Multisite New User Form vs Block New Admin