When viewing the details of a cell, the edit links for feedback and evaluations appear for all reviewers and evaluators, respectively. When someone other than the original owner attempts to edit these materials, an error is thrown. The check should be that them item would be editable and that the current user owns it (or possibly, is admin).
If true group authoring of these items is actually needed, a feature should be requested. There is significant complexity in implementing shared ownership of forms, in terms of permissions and workflow, as well as raw technological constraints.