If you have shared a Google sheet with colleagues, then open the sheet, you will be able to see at a glance if anyone else is working on the sheet at the same time. You can chat with collaborators using the pane on the right-hand side of the screen. 

If a colleague that has editing access to the sheet opens the sheet, you will see that another user is viewing the sheet in the top-right corner of the screen. Position the mouse over their image to see their contact card. 

You will be able to see any changes that the other person makes to your sheet immediately, and will also be able to see the exact part of the sheet they are working on. 

You can chat with all collaborators currently working on the sheet.

If you receive a chat message in a sheet while the chat window is closed, a red indicator will show on the chat icon:

Note that the chat is not saved in the sheet – if the sheet is closed, the chat will be lost.