How to show particular menu for a group of users editors for example?
content, as option in the category, specific to a user on wordpress?
example, a letter from the administrator to a user, where only the user can read this letter when you log in?
It somewhat depends on whether you want the menu, letter, etc. to appear in the Admin panels or on a WordPress Page (which only the user can see). Both are possible and generally require you to program it in PHP yourself, typically through a plugin or template that you write yourself.
But, first, check through the Plugins and see if there is anything that will do this for you. WordPress itself does not provide this functionality directly to you without you programming it. http://wordpress.org/extend/plugins/
For the limited work I’ve done in this area, I wrote it all myself in php.
I really do suggest you do your own searches at http://wordpress.org/extend/plugins/ and try and choose a plugin that looks promising, give it a try by installing it and testing it fully. As many times as it takes to find something that will work for you.