The "Add Multiple Users" plugin is a free plugin that we have used for many years to do the following jobs:
- Upload spreadsheets of users to create logins in bulk;
- Transfer users between blogs in bulk by selection from a tick list
We have discovered that the "Add from network" function (which is used to transfer users between blog) does not work in sites with over 1000 users. When attempting to transfer users it comes up with an error "Unknown Request"
Unfortunately, as this plugin is no longer being developed there is no prospect of the problem being resolved. The only solution we can find is to delete legacy users from your site to bring the total number of users to under 1000.