Contact groups that you have access to (either that you have created, or that have been shared with you)

_idStringUnique ID
userIdStringUser id. Will be yours unless this is a shared list, then it will be the user id of the creator. Pass expand=userId on the querystring to expand this to include the name and email of the owner.
groupIdStringIf this group is associated with a google group, this will be the google id.
nameStringName of the contact group
count (meta-field)NumberNumber of contacts in this group. This is a meta-field that is only returned if you pass expand=count on the request querystring.
sharedObjectSee format.