Collaborators are users who are given access to someone's work diary. 

E.g.: Let’s say you have a contract running. You would normally review the work of your team member by checking out their work diary. When you invite a collaborator they have access to view the work diary on the contract you selected. The advantage of this is that if you won’t be around for a few days, your assistant can step in and make sure work is getting done.
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