If you have already sent your invitation, the options can be adjusted from the Manage Invitation page:
To adjust options in the Punchbowl app, first tap the image of the invitation on your dashboard. On the next screen, you'll have an Options link in the upper-righthand corner. Tap here and then select Invitation Options from the dropdown menu.
If you're in the process of setting up your invitation, you can adjust the options by clicking the Invitation Options button at the top of the Add Guests page.