A script or an auto-entered calculation can be used for this.
Make your All check box the format for a separate field from the checkbox group field.
Define this auto-entered calculation for your checkbox group field:
If ( IsEmpty ( AllcheckBox ) ; self ; ValueListItems ( get ( FileName ) ; "Name of your checkbox group value list in quotes" ) )
Then clear the "do not replace existing values option" for this field.
A script can use the ValueListItems function with a set field to do the same thing and you can set up your Allcheckbox field as a button that performs this script if you choose to use that option.
Thanks you very much!!! What can I add to formula to uncheck when I uncheck the "allcheckbox"?
Change self to "".