I looked at my "Family news" section & saw another user tag some people. Some of the people weren't on my tree yet. When I looked at the tree I saw that these people haven't been added to the tree - seems that these people have only their profile page.
I wanted to associate them to a relevant place on my tree but there is no such option. I have only the following option under "actions" box for these users: Ancestors, Descendants, Relationship to me, Photos , Timeline.