I am running Sierra 10.12.3 on a new iMac purchased in Jan 2017. We are running on an exchange network and I'm trying to re-create my groups in Contacts. I create the Group, search for the contact card on our exchange directory, drag it to the group and agree to import the card. I review the list and everything looks good for about 10 minutes. After that I can go back in and all or some of the contacts are missing. How do I make them stay in the Group as I rely heavily on them for booking committee meetings.