If it is not convenient to send a direct link, friends can be given a Referral Code unique to the referring player, so that they can be credited and receive rewards.  This code can be found in the invitation menu; the button that appears next to it can be used to copy the code to conveniently paste into any text or message.

Players receiving the code must enter it in their own invitation menu before reaching Level 10.  Once the new player completes Level 10, the referral will be credited.