Client group based on custom client field?

Hi, Can anyone tell me how it might be possible to add a client to a group, based on a custom field?

For example, you create a custom field "I'm a Web Designer" with a check box. If the customer fills that box at signup or in their profile, it adds them to a group "Web Designers".

This could be useful for offering out client projects to customers who are web designers by emailing that group.