Riffle populates common cost groups for you at the start of the project. You can add any missing cost groups by pressing Add a cost group at the bottom of the page.

You'll need to populate all of your own line items in a cost group. You can do so by importing an invoice or pressing Drop an invoice here (or press) to add a line item at the bottom of the cost group.

Remove any cost groups you don't need by pressing the Delete cost group icon on the far right side of the row.

Remove any line items from a cost group by pressing the Delete line item icon on the far right side of the row.

Related: Manage Your Job Costs