Feature/week5 - invite friend and setting mailgun

I am not follow the solution of invite friend. 
I suppose the user has the never-change and unique token for invite other.
User can use the token to invite anyone by manual add the token in the log-in path.
This method maybe can use to implement a feature: 
  user can getting reward if invite more people to this site.

Something needs to be careful about this approach? 
And have any suggestion to my implementation?
Thanks.