The easiest ways to add guests is to allow Punchbowl permission to import your iPhone contacts. To import, after you finish editing your design continue to the Add Guests page and you will see the following prompt:
Select YES and your iPhone will prompt you to allow Punchbowl to access your contacts. Click OK.
After allowing access a list your will appear. Use the Search field or simply scroll to select the Contacts you would like to add.
When you're done, tap the "Add guests & continue" button. Don't worry, you can always come back to add more guests later.
Need to allow Punchbowl access to your Contacts? See this help article