In my experience, the rules for groups are driven by business logic rather than computational ease. That is, there are rules somewhere that determine who can see what and these rules can be based on law, position in the company, clearance level, or personal preference of the rule-maker.

If your concern is for performance, I wouldn't worry too much until you see the performance problem manifest itself. I've worked on systems that were simple user->access thing before that scaled pretty well. (Caveat: these were on fairly large hardware.) I think you may have some space before you hit a wall.

If your concern is maintenance, I think maintenance is easier if the maintainer can hang the group on something tangible. For example, the Accounting example provided above. I think it would be very difficult to maintain groups based on some optimization paradigm. In fact, you would probably need to re-group over time as people moved around and the landscape changed.

I'm not aware of any common solutions to the problem. My guess is that it's because the solution is often specific to an organization. So if you hit the performance wall, you may need to work with the people who decide on report access and determine how people can be grouped more efficiently.