To add a customer to a check (new or existing) you will need to follow these steps:
1. Select the table you wish to add a customer to.
2. On the order entry screen tap the add customer button towards the top (the plus symbol)
3. A new seat/customer has been added
4. Add the items you want to the seat selected
5. You can reopen the check to see what every seat ordered, including the one you just added.