Once you have set up your Family Sharing master device adding an additional person is very easy. Navigate to Settings > iCloud > Family and tap the "Add Family Member..." panel. This will open a new window. From here just type the email of the person you want to add.
To change the credit card associated with you Family plan navigate to Settings > iTunes & App Store and tap "Apple ID:. Tap "View Apple ID " from the pop up box and enter your iTunes password. This will allow you access to your iTunes account. From here tap "Payment Information" then edit your credit card information.