In order to achieve that affect, you need to use Custom Values. They allow you to define your own pieces of data for Grindstone to keep track of.

1. Begin by clicking on the Profile menu and selecting the Configure option.
2. Next, go to the Custom Fields tab.
3. Here, create a new Custom Field called "Client."
4. Click OK to save your changes.

Now, "Client" will be available in the Column and Group By functions. You can assign values to the "Client" custom field by modifying tasks and flipping over to the Custom Values tab.