Why can't I change names on faces in pictures shared with me
When pictures are shared with you, you can see the names the owner has added. You cannot, however, edit names or make changes.
If you've uploaded several dozen pictures and added names, when you share those pictures, the guests you share them with can only see the names and reference portraits for the people you've named. They cannot make changes such as editing names or updating reference portraits.
Further, if they have the same person named in their own account, the pictures you've shared with them will not be added automatically to their albums of that person.
Imagine siblings, Alice and Bob. They both have Ponga accounts and have both added family pictures. In all likelihood, they both have pictures that include their parents. When they share pictures, each will see the name the other sibling added for their parents. They will not be able to edit the Face Details pages on pictures their sibling added, however.
Best Practice Tip: If both Alice and Bob named their mother "Mom" and share their albums of all pictures with their sibling, they'll both end up with two albums named "Mom." That can get confusing.
A good practice is for either Alice or Bob to create a new album of pictures to share with siblings and name it something else, for example, "Parents." They might toss in all pictures they think their siblings might appreciate of their parents. If Alice does this and shares her best pictures of her parents with Bob. In return, Bob can add a few more, perhaps focusing on the pictures Alice doesn't already have. When they open a picture including their mother, it will still say "Mom." When looking at the picture, the by line in the upper right corner will include "by Alice" to make it clear whose picture it is.