Granular ACL

Hello, I’m completely new to WP. I searched a bit around but unfortunately I haven’t found a solution (I’ve problem with the search feature of this site, since when I search it hangs on, with no results…)

My problem is that I want my users to have the ability to write posts but only in a specific category.

I read docs about users roles and found the plugin for roles and capabilities, but didn’t found how to give a group the ability to post only in a specific category.

Does someone has an hint on how to achieve this?

Thanks!