To add the same PO number to all the jobs contained in a group at once, you have to go in the details of the group:
by clicking on a coloured dot in front of a job in your list of jobs
or by clicking on the name of the group on the left when you're in the details of a job
Once you're in the details of the group, click on the little pencil next to 'Purchase order' to add a PO number:
This PO number will also automatically be added to each job separately.
If a PO number had already been defined at the job level for a job in particular, it will be replaced by the PO number you define at the group level.
You can't edit a PO number defined at the group level in a job in particular. You can only edit it in the details of the group.