The EntryID for a recipient object will encapsulate the contact EntryID. Use
OutlookSpy (www.dimastr.com) to examine both EntryID's for a recipient and
the related contact and see where those patterns are. Then just strip off
the rest of the recipient EntryID string and you have the EntryID for the
contact. In general you would strip off the left-most 25 characters from the
recipient EntryID and start with the 26 character to get your contact
EntryID.

Of course that wouldn't work for a one-off DL member but you can check the
OneOffMembers property, which is a PT_MV_BINARY property. You can see that
in OutlookSpy also. In fact, the Members property is also a PT_MV_BINARY in
MAPI terms.