[hlieberman]

If it's salted, you can't share it with a third-party and determine who your customers in common are. (That's the point of the salt; to mean that my_hash(X) != your_hash(X)).

[OutOfHere]

You actually can join it when the salt provider is a dedication shared entity. The entity rehashes the data of both organizations to use a shared salt. That is how different organizations join hashed data.