You basically add them as the parent/spouse/sibling to a profile you manage, depending on their 'role' so to say.
For that you use their Wikitree ID. That is the part at the end of the URL. Your ID is Whitfield-2407
Copy that in the box on the top of the edit page.
So for instance, if you want to add a mother to a person, and the mother already has a profile. Go to edit, click on the mother, copy the WikitreeID into the small box on the top right side of the page and save.
This works if the profiles are public (open). If they are not public you will need to ask the profile manager via a comment to do it, or join the trusted list.