Tap or click to add a bill from your home screen, add the friends you'd like to include on the bill to the 'With you and:" line at the top of the "Add a bill" form. 

To be sure you're not adding the bill in the group double check that you see 'No group' on the add a bill form. If you see a group's name tap/click on it and select "No group" from your list of groups.