Unfortunately, it's not possible to use a group of Xero Contacts.
However, you can select specific Contacts that each Requester/Approver will be able to use.
This is how it works:
Open the Requester/approval matrix and select the option "Contact that matches" / "Supplier matches".
Click on the box to select the Contacts/Suppliers that you want to be available for each Requester/Approver (those Contacts are taken from your list of Xero Contacts):
In addition, ApprovalMax supports the copying of rules and restrictions from one workflow participant to another.