CSS is dependent upon building selectors which match elements in the HTML structure of the web page (e.g. right click and choose Inspect element with your web browser to view the HTML).
In some cases, depending on what you're working on, it may just so happen that the HTML has a class or id related to "user groups" that you can use. But most of the time, it doesn't.
Much more likely though, if you're talking about forum posts or comments made by users, their username may appear somewhere in the HTML, so you can build selectors based on usernames. But you would have to do that for each and every single username who you know happens to be in the user group.