Add Collaborator Column
To add a collaborator column, scroll as far right as you can on your table, then click the large + button. Then select the 'Collaborator' type.
Once you add the Collaborator column, you can choose a collaborator by double clicking on the cell. Depending on whether you choose to select one or many collaborators, you will be able to select one or multiple collaborators.
Note that if you add a collaborator column to your sheet and share the form publicly, then users who view the form will be able to see the collaborators as well. Of course, if the field is hidden, they will not see the collaborators.