I am designing a trading card game for mobiles, with the possibility to add cards to your collection using Gems, aquired through victories and inapp purchases.

I am thinking to increase the spread of the game with a tracking system on game invites, enabling the user to invite a friend to play the game. If the friend doesn't own the game client (which is free) he will be offered to download it. If he joins the game, the original player earns X amount of gems as an reward.

There can only be one player per mobile device, which should rule out some harvesting.

My question is, how do you think the structure of this would be recieved ? All invites are mail based, unless the player already exists in the game world (then he gets a ingame invitation.)

I have set a flood filter, so a player can only invite a friend (without the client installed) once a month.