You are doing a good job with your English 
The set-up for this is tricky even when English is your native language.
Yes, you will want to follow this guide: Understanding groups and category permissions (security settings)
Close!
If it helps, here is what I have in my theme setting for the demo:
junk~restricted category preview~This category is only available to members of the "joinable-group" user group. Select for more information.~https://theme-creator.discourse.org/pub/logged-in-user-access~joinable_group
And how it relates to the description of the theme setting:
| Theme Setting Part | Value |
|---|---|
| category-slug | junk |
| Preview Name | restricted category preview |
| Preview Description | This category is only available to members of the "joinable-group" user group. Select for more information. |
https://example.com |
https://theme-creator.discourse.org/pub/logged-in-user-access |
| group | joinable_group |
And here is a diagram to show how the different parts relate:
In the image, “ 
restricted” is the real category that can only be accessed by the joinable_group user group (and staff by default).
