Breaking it down, when we say:
George, this is Joan
We are addressing George, but we are introducing George and Joan to each other.
introduce (verb) make (someone) known by name to another in person, especially formally:
I hope to introduce Jenny to them very soon.
Unless one of the two parties is behind a pane of one-way glass, or George and Joan are in a room where music is blaring so loud that only one person can barely make out what their friend is saying, as soon as one says, "George, this is Joan," then George becomes aware of Joan's name, and Joan also becomes aware of George's name.
Sometimes, for the sake of politeness, one might "complete" the introduction by saying the names in the opposite direction:
George, this is Joan; Joan, this is George.
Or, more tersely:
George, this is Joan. Joan, George.
but some might find that second part redundant because no information is given. Still, the longer version might seem more polite; otherwise, Joan might feel left out of the introduction.
Asking which person is introduced to whom is sort of like asking, during conception, does the sperm fertilize the egg, or does the egg get fertilized by the sperm? The introduction happens, and George and Joan both learn each other's names.
If, on the other hand, you only said to George, "This is Joan," then George has been introduced to Joan, but Joan hasn't been introduced to George yet.