To add a collaborator as a bulk action to several staff reviews, you should start by going to Settings from the left-hand sidebar.
Then click on the Review dropdown, and select General. This will take you to the General settings, from there you need to select Bulk Actions from the list of categories on the left.

When on the Bulk Actions page, select Collaborators and click Start
Select the date range of the reviews to which you wish to add a collaborator

Click Next
Select the members of staff you wish to add a collaborator to their review

Click Next
Select the collaborators you wish to add to the staff member reviews previously selected and select what functionality you wish the collaborator to access. View is a view-only option, whereas manage allows the collaborator to carry out a specific activity

Once selected Click Next

The checked reviews will be updated with the selected collaborators and related functionality.
Click Add collaborators