Guest viewers can view projects, approve/reject creative, and download creative (if enabled).
A guest viewer sees the project exactly in the same view. This is handy if you need a co-worker or client to know exactly what's going on but make sure they can't interact with the content in any way
Guest members can do all that too, plus: create custom collections, reorder images, upload new images and edit descriptions.
More information on how to make use of sharing here.