Can I activate my plugin only for those registered on the site?

Okay, thanks!

I’ll check it out right away, but I already have a question.
Why can’t I define a user group as not logged in? Other forums allow this. I mean, they give them permissions (like ipb, mybb, Vbull)

What if I want to write my own plugin that only activates for anonymous users? Is it enough for me to have a css class or something else?