Every spreadsheet needs to be accepting connections.
Choose Accepting Connections from the Connections menu. The worksheet should then say accepting connections in the upper right hand corner. You will notice that in your Xess spreadsheet that you have a new menu with the name of the other user's workstation. If you select a section of your worksheet you can then choose send values from the workstation menu and those values will appear on the other user's workstation. Note that this CAN cause complications if more than one person is working in the same section of the worksheet because cells can be overwritten.