I wouldn't encourage people to take actions with a physical reward as their motivation.
First, it gets you the wrong type of users, because people completely uninterested in your service will get others to sign up just because there is something in it for them (or create fake accounts, bots, etc...). Second, if you use this approach it's hard to gauge real interest in your service. It should be interesting enough that people will want to show their friends with no reward, if it's not I'd solve that problem first.
If you feel you must incent them in some way, maybe a free month of the service, a "Founders" badge or something else that relates to your service but is intangible might be a better route.
First, it gets you the wrong type of users, because people completely uninterested in your service will get others to sign up just because there is something in it for them (or create fake accounts, bots, etc...). Second, if you use this approach it's hard to gauge real interest in your service. It should be interesting enough that people will want to show their friends with no reward, if it's not I'd solve that problem first.
If you feel you must incent them in some way, maybe a free month of the service, a "Founders" badge or something else that relates to your service but is intangible might be a better route.