Group Reservations contain one collective invoice for all parties, but there is the option to separate invoices through these settings.
From the Reservation page, click on the Unit and Edit>Change Guest. If there are multiple units, then these same steps will need to be repeated for each unit.
Select either 'Group Members' or 'All People', and search for a person or click 'New' to enter a new person.
Click on 'Transfer Charges' for a separate invoice on the same group reservation. 
Continue in the top right to apply.
For further guidelines, please refer to our Knowledge Base article Multiple Payments on one Invoice.