Friends joining the same game that you play is already awesome but even better are rewards for invited players!
The first colony of a referred empire gets placed within the same zone as the referrer's empire. In addition, the referrer receives precious Essentia for every University level beyond four that the referred empire reaches.
To share your individual referral URL, visit the Essentia window, click Invite a Friend, activate the button "Generate URL" and spread it wherever you may, or use the built-in function to send an e-mail. All the recipient needs to do is to found a new empire via the referral link.
Happy referral time!