A search with this URL:
netted a couple of possibly relevant how-to articles:
The first article mentions Apple Mail's Smart Groups function, which is available in Mountain Lion but not Leopard, unfortunately. This works similarly to Smart Albums in iTunes, where a domain name or other criteria can be used to automatically generate Groups for Mail. With Mail in Leopard, Groups are more manual, as you noted.
If you use Mail or anything else on your MacBook for your business, I hope you're using Time Machine to back things up!Out of curiosity, what model/year of MacBook are you using?