Many devices and applications support groups in vCard 3.0 today by putting a delimited list of groups into the "CATEGORIES" field of the vCard. If you just had a simple textbox to edit the "CATEGORIES" field in the webapp, then the fruux webapp would work better with existing devices and applications. Even better you could allow filtering of contacts by the delimited items in the "CATEGORIES" field. For ideas look at how it is done with the grouping in the ownCloud demo.