When you share a sheet with other collaborators, you will be able to start public and private discussions in a small chat windows displayed in the bottom-right corner of the page. In this window, you can discuss your project tasks.
The discussions tab allows you to discuss your project in a larger screen. To notify a collaborator in a discussions, you can @mention them by typing the @ symbol and selecting their name from the dropdown menu, or you can click the @ icon in the comment box. When you @mention a collaborator, they will be notified that you mentioned them in that record.