In your account at accounts.play.eco there is a section invites where it shows your current invites. There you also have a insert-box where you can enter an email to send an invite to. The invited person will get a link and can create an account that has Eco as well.
I thought so as well but I could not find any option there except buying more invites I never send an invite out nor used it myself (since I already owned a Steam version of Eco). Now I would like to convice some friends of the "awesomenes" of this game but could not figure out where to send the invites.