I don't know much about group accounts but according to the book "Getting Started With Mac OS X Leopard"...
Creating a Group Account
A group account is a new feature in Mac OS X Leopard that allows you to set permissions for a number of accounts from a single point. For example, if you have a folder of files that you'd like a group of family members to have access to, you can create a group that contains all of the family members accounts.