Each account can only accept one game invitation.
Please make sure your device has the game installed. Then return to this page and click [Launch Game] to complete the invitation.
Alternatively, you can go to the game's Main Menu, click [Your Name], then click [Invitation Code], and input your friend's invitation code (5JJRN16BBP) to complete the invitation.
After establishing an invitation relationship, the support-elementron of the inviter will be able to assist you in battle.