Proper usage of User Groups with Group Policies is a must. First create User Groups based on the job functions within your company. Consider the security of each job function and even if it does not conform to a job function make User Groups for specific tasks or needs. Assigning individual users to Group Policies is something you do not want to do.
It may seem easier to have just one User group for everything but as people come and go, or even move to different positions withing the company they will need to be able to change User Groups to accommodate their positions. If you have a blanket group for high security and another for low security you may find when a position changes that a person needs some policies bases in one group and some in another. At this point you would not have the same flexibility as you would have if you had different groups for individual tasks.