There is no where to enter friend-codes for players who already play the game. I have searched the forums and emailed support. I have received 3 different answers from the support team. It seems no one in support can tell me where to enter friend-codes.
Support sent me a link:
Each player can have up to 20 voucher-friends. Additionally players can have up to 20 ingame-friends.
If you give your friend-code to players who already play the game for some time, you also will earn some vouchers when they reach a new hero level, but you will earn A LOT MORE vouchers if you invite new players to the game.
There is no where to enter friend-codes for players who already play the game. Either you should fix this bug in the game or you should up the forum.