To set the start and end dates for each group of tasks in the projects we need to link the cells from each task in the overview project to those of the projects in the other workbook.
This use to be a fairly complicated process but the right hand mouse button (RMB) and the 'Define tasks for group/overview' option now make this straightforward.
Select the group/overview name in the overview.xls ProjeX sheet with the RMB and select the 'Define tasks ...' option from the list.
This brings up the following dialog asking us to select the tasks we want to 'group' in this single overview task
If we click on the '-' icon the dialog minimizes and we can switch to the Project1 sheet and select the tasks to be used.
Expand back the full dialog having selected the range of tasks to use and okay out of the dialog.
This updates our overview task with the start and end dates, duration and the % completion of the group of tasks selected from the sub-project. These are direct Excel links so when we update the sub-project the overview will be updated the next time it is opened.
There is currently a limitation with ProjeX that the overview sheet must have the same number of padded columns as the individual projects. This will be removed soon.
We can repeat this for the rest of the overview tasks.