The Active Collaborator feature allows you to see when another user is working in the same rundown but more importantly, it shows you what cell they currently have selected and if they are in "edit mode" on that cell.
When another user is working in the same rundown, you will see their user icon at the bottom middle of the screen and the cell they currently have selected will have a highlighted border. If they enter edit mode, the entire cell will be highlighted.
Note: Make sure to avoid making changes to a cell that another user is working on. Doing so can cause one user's contribution to be overwritten.