Yup, you can - You can invite him through some channel from the game (Go to Invite and choose some option like WhatsApp, Viber, SMS or similar).
Other option is to set your Association to be "Join by Request", tell your friend the name of Association (or the tag) and after he send his request add him and turn back your Association to be Closed
Hope that helped
Great, thanks! I'm mostly using the web app, where are can invite only Top Eleven o Facebook friends, in the mobile app now I see sms, whatsapp and other possibilities :-)