Sharing Outlook Contacts: Adding contact to a Contact Group in another users folder

I have access to another user's contacts. I need to create a group for the other user.

The contacts that need to be in the group are in the other user's contacts.

I have created the group and was able to add some contacts to the group, but I cannot add the others that need to be in the group even though they are in the other user's contacts. Why is this and how do I resolve this issue?