If Luca Brasi was so reputed to be loyal to the Don, why wasn't he ever officially a part of the Corleone "family"...or was he?

Did Don Vito not trust him enough to let him join his family?
I don't recall anyone in the top brass of the family mentioning him in that capacity.

"Now, that plane goes to Miami."
"That's right. That's where I want it met."