What I have now:
1. I've a new 'Category' , marked as 'Private Node'.
2. I've a new forum inside this category; and I want select users from my site, who belong to different user groups to have special permissions to this forum.
It looks like the best option for me - is to create a special 'usergroup' and then allow this usergroup to have access to this private forum (inside private category).
I've been trying to add node-permissions to users individually; but it's turning out to be totally complicated.
Do you think creating a new usergroup is the only option?