Filters whether to show the Add New User form on the Multisite Users screen.
network_site_users_after_list_table
Fires after the list table on the Users screen in the Multisite Network Admin.
show_network_site_users_add_existing_form
This filter is documented in wp-admin/network/site-users.php
network_site_users_created_user
Fires after a user has been created via the network site-users.php page.