In Striker Manager 3 inviting friends will give you incredible rewards. Every week there is a promoter ranking for the users than bring the most friends to the game and the season ranking gives unheard of rewards.
To find the promoters section go to community and then promoters.
To invite a friend use the code that appears under "Your promoter code" or use the promoter link.
You can check each week the ranking or check the seasonal one.