The Enclave has been about for a nebulous eternity, so yeah, some of Eusine's ancestors could have definitely been members. Jack is pretty new himself so he wouldn't have known them or anything but if he looked up Eusine in the records what's what he'll get.

Jack doesn't advertise being Sorcerer Supreme pretty much anywhere outside of Enclave business because it honestly makes him uncomfortable, so in general real world business he just goes by Plaid Jack Clarity or Jack the Plaid.

He's shows would probably be fairly well known in the area of down he does them - they are mostly a children focused thing, but he's popular enough there that he'll have random people recognize him and give compliments. They definitely could know each other through that!