I have been looking for a solution and definitively you are right on both things.
1- It is not possible to create users in a category context role (This is confusing, because when you define the permissions, the "create users" option is configurable but actually, this option is not aplicable in a category context. ). So I made what you recommended me: create a new role with permissions for create users and give it to the category admin in the system context.
2- Automatically filtering of users by category is neither possible, like you explain me correctly in your last message. So I have started to change the code to filter the users for every category admin. ( I use "user profile fields" to make a relationship between users and categories. I don´t like too much to modify the code, for mantaining the stability, but it seems there is not other way and is working right for me +-.)
Thank you very much for all your support!!
