The system group ID for this group, if it's a system group.
If the group is not a system group, this is null. Otherwise, it is one of the four system group IDs: gdata_contacts_group_contacts, gdata_contacts_group_friends, gdata_contacts_group_family and gdata_contacts_group_coworkers.
If this is non-null, the group name stored in title will not be localised, so clients should provide localised group names of their own for each of the system groups. Whether a group is a system group should be detected solely on the basis of the value of this property, not by comparing the group name ( title) or entry ID (id). The entry ID is not the same as the system group ID.