The clan I am in received a Diamond invite. We have not decided whether to accept it or not. If we choose to remain in Platinum, and lets say we stay there for a while, and perform poorly, will our Diamond invite be withdrawn/expire?
As of right now no and you can opt in and opt out as much as you want . Don't forget that when opting in you have to do it before the roster lock for the bronze through platinum divisions that way you don't get placed on there.