dfdfdf said:

I can't figure it out how to do the following things:

1)) I have multiple users who I want to assign to be able to post in different (for each user in other) categories (and other categories are restricted). How to do that? (I worked it out for Pages, but not for Posts).

Here's the simplest config to accomplish that: Give your users a WP role of Subscriber (or a renamed copy of it with no post editing capabilities) and a Category Role of "Post Author" in desired categories.

2)) I want a single user to be able to see a plugin which he has to manage. How can I let him see this in the backend (it's easy signup)?

I would check with the Easy Signup author(s) on those requirements. If the need is to block option access for some users, the Adminimize plugin may be helpful.