Collaborators may include peers, teams, fellow researchers, supervisors, students, etc.
Collaborators can be added (or re-added) from two places - the Landing page or the Analysis Page.
Add collaborators from the Analysis Page
Add collaborators from the Landing Page
From the Landing page:
Open the context menu: Right click on the file you'd like to add collaborators to. The Select Invite Collaborators pop-up window will open.