After you've saved your product bundles they will be clearly marked with a 'BUNDLE' tag on the right side, below the price.
Since the app treats them as single products, they take up only one shortcut grid slot.
For example, on the screenshot below 'Pizza special' icon is a product bundle:
Once you select a bundle, all you need to do is select one product from each group, unless all groups consist of only one product. In that case, there will be no need to select anything, they will be automatically added to the cart.
Tap 'Show products' to reveal the bundle content in the cart.
Note: Product bundles are available only for app version 3.2.0 or higher, lower version apps cannot access orders that include bundles.
At this moment it is not possible to include product bundles in online ordering (Click & Collect, Order & Pay), nor use modifiers with product bundle items.