I don't see address groups as a feature in Roundcube out of the box. Is there a plugin or patch available that would add the feature? I have reviewed the Plugin Repository on the Wiki page, but didn't see anything that described what I was looking for. It is possible I might have overlooked it if it was there.
Contact groups are planed for RoundCube 0.4 beta Roadmap ? Roundcube Webmail (http://trac.roundcube.net/roadmap)
According to a post in this thread (http://www.roundcubeforum.net/requests/5334-addressbook-needs-more-fields.html), groups are planned for version 4.