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 the group in question in your list of groups
or by clicking on one of the jobs in the group in your list of jobs
Once you're in the details of the group, click on Edit in the Billing panel. You will be able to add a PO number that will be automatically applied to all the jobs in the group:
Please note:
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.