How do I add photos to a "For You" Album on my iPhone 6s?

Photos will make collections of pictures that it thinks you will like using its own criteria-- maybe based on times or locations or subjects. Since the images are selected by Photos, you can't change the collection or add to it. But you can make your own by selecting some pictures and using the More (•••) menu to Add to Album. You can use the + button to make new albums for your own memories collection.

When I see a collection that I like from Photos, I sometimes select the pictures the best from there, create a new album, and add others that seem appropriate. So I use the "For You" and Memories things more as suggestions for making my own albums.

I hope that helps…