Currently only the "owner" of a shared album has edit capabilities on the images in that album. If user "A" wants to allow user "B" to view and edit some of their images, they'd need to set things up correctly using the new "contribute" feature. That requires user "B" to create an album which is shared with user "A" and has the "Can Contribute" option enabled when they send the invitation to user "A". On receipt of the invitation, user "A" can then add/contribute their images to the shared album....a copy of those images are transferred to the shared album which will then be added to user "B's" library (i.e. it affects the cloud storage allowance). Once that has been done, user "B" can now edit those images, and user "A" will be able to see those edits (though the originals are still in user "A's" library and can be separately edited by user "A" and invisible to user "B".