There is currently no way to define groups in the contacts module afaics. I'm not sure if that's somehow standardized with vcards or not, so no idea what the best way to implement it would be.
In any case, it would make sense to add support for it to the module in my opinion. Maybe @untitaker could share his opinion, as I think he worked with calendars and contacts (outside of RS) quite a bit in the past.
The easiest way would be to just create subdirectories for address books/groups. You could check out the albums in the pictures module, which is doing basically just that. Then just add a (optional?) group param to the
get functions for example.