As mentioned in this post about organizing projects, a group may be considered as a project of its own, but still part of your existing project. Groups are a convenient way of organizing your data and using them as mini-projects is just one way of how you may use them to boost your productivity.
Let's take the example of a catering company who has a few regular customers, one being a company named Greedy Consumer (yeah, I know, clever name). The catering company takes care of their customers' orders using a project named "Food delivery".
Since they handle a lot of data for the Greedy Consumer, as recipes, messages and milestones for when the customer needs food delivered, they create a group named "Greedy Consumer" that holds all these objects.
It's easy to navigate inside this group, browsing only through data related to Greedy Consumer, because when a group is selected in one of the pages, the selection is persistent. For instance, if on the tasks page they select the group "Greedy Consumer", subsequently selecting the milestones page, it will show the milestones for this group.
This is just a hint on how you may use a feature in Teamness that I wanted to bring along with our sincere wishes of success with your projects in 2009, hopefully managed with Teamness.
Happy New Year!