Consider the following tables
tblPerson(PersonId, GroupId, Forname, Initial, Surname, DOB, etc)
A Person belongs to a group (like employee to employer)
In the past we have used the following table such that a Supplier is always a Group and Owner, CurrentKeeper and Installer are a People.
tblItem(SupplierId, OwnerId, CurrentKeeperId, InstallerId)
But now we are faced with the fact that Supplier, Owner, CurrentKeeper and Installer can be a Person or a Group!