While adding a collaborator as an editor means you can work on a session together, there are some actions which only the session owner can do. The key actions are:
Adding, extracting or deleting a day to your session
Delete the whole session which they'd created
Extracting a day from the session or merging it with another session
It’s always the session owner’s user rights which determine how a session can be shared. A session must have an owner but this cannot be transferred to another user.