Google Docs allows you to upload and create your own spreadsheets. It also allows you to share it with other people who can either only view the document or edit it as well. When the person is viewing or editing the document, Google docs notifies you with a colored box that another user is viewing the file. If you click on the colored box, a chat window will open on the right hand side, through which you can have live discussions with the other viewer who is viewing the file. The two of you can discuss things related to the file. Furthermore, multiple users can work on the same file in real time. Therefore, make sure to communicate the changes to the other viewer in order to avoid repetition.
Follow this step by step Google Docs tutorial to learn How to Chat in Google Docs Spreadsheet.