The account tab is where the "Refer friends, get $7" option is. It's highlighted in red so it stands out more in the menu.
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The refer a friend page has options to refer through email, text, Facebook, Twitter, and Facebook messenger. The email option has the biggest button. There's also a section below the share buttons for tracking your referrals.
Selecting the email option opens up a list of your contacts. Once you select who you want to invite, you continue by pressing a large "Send Invites" button at the bottom of the screen.
The invite is sent in the background via an email. You don't see the contents of the email. "Invites sent" page lets you know you can come back to check the status of your invitees.
It also asks you to keep spreading the word, and also includes share buttons for SMS, Facebook, Twitter, and Facebook Messenger.
Once you're back on the refer a friend page, you can see it's been updated with the status of the invite you just sent.