There are several ways to start a group lunch:
Option 1: (not logged in)
You will see the "Start your Group Lunch" button in the home page and you may follow through after clicking it.
Option 2: (logged in)
You will see the "Start Your Group Lunch" button at the top right corner of your page.
Option 3: (logged in)
Go to your calendar, pick and click the date for your lunch and you'll see the "Start a Group Lunch" option.
Here's a short clip for visual reference: