Follow these steps to add a modifier group to an item:
1. Log into the back office.
2. Go to products and items.
3. Open up a menu item under products and items using the blue pencil icon.
4. Select either optional modifier, included modifier or mandatory modifier. (these are set up the same way)
5. Click the add modifier group, then select the group you wish to add to the item along with specific items or prices.
6. Once finished hit save and publish.