Pass the code to your friend and let him enter it during signing up. Press on Copy button on the screen and paste it in email/sms to your friend. Let him do the same (copy/paste) to avoid any common typos (0 - O, 1 - l, etc.).
To get to the code, sign into your self-serve page, and at the bottom right, it's part of the cycling messaging.
Tap the COPY button when there. (a screenshot is certainly helpful, @Metal1967, however may not allow the receiver to discern certain characters). Then, send the code via SMS or e-mail to the person you're referring and have them PASTE the code into the field on their activation page.
Failing to do this may result in certain characters being confused for others (such as zero's and O's, and one's and I's, etc...)