Hello,
Sorry for the delayed response. I believe the issue is you would have to save the record first before you can filter. The field values would be coming from the database tables and not the grid. You can add checkboxes to the grid from Step 1 in the grid edit screen. There is a 'Checkbox column' option there that you can check. Then you can have a button that loops through the grid finding the checked customers and add them to a list or save a field on the Contacts record. The action would be a 'Start Loop Through Grid' and there will be an option for checked rows.
Below is the action for the Invoice Selected... action on the Work Orders page.