Default group for users?

I'm curious to know from the admins of Linux networks what their policy is for groups for their users?

Do all of your users have their own personal group and then are added to additional groups for the privileges / access they need?

Or do you not have personal groups and simply assign users to a default group and then add additional groups from there?

Or do you do something else entirely?

If you wouldn't mind providing a brief overview of why you use the method you use that would be appreciated. Such as, if you do not use personal groups is that simply to cut down on the number of defined groups or some other reason?