In Manage | Database | relationships, make a new table occurrence of YourTable by clicking it and then clicking the duplicate button (2 green plus signs). You can double click the new occurrence box to get a dialog to appear where you can rename the new occurrence box.
We have not duplicated a table. Instead, this is a new reference to the same table already present in your database.
Add it to your relationships like this:
YourTable;:anyField X YourTable 2::anyField
Drag from any field in YourTable to any field in YourTable 2, then double click the line this creates and change the = operator to X.
Now add the summary field to your layout, but select it from YourTable 2 instead of YourTable and it will show the grand total based on all records in Your Table.