As noted in the comments, "going out" to a place means going somewhere that is thought of as "away from the perceived centre of the universe," as Edwin Ashworth expressed it. To "get out" to such a place means to get there.
One could replace, "if you ever get out to California..." with, "if you ever make it to California...", or "if you ever are in California...", although the latter omits the allusion to the idea that California is a place one "gets out to," because it's somehow a far off place.
So, the closer the person being invited to visit is to California, the less likely it is that the invitation would include "get out to." Since the friend being invited is from New York, the use of "get out to" is particularly apt. However, if the invitation began, “I told my friend in Oregon", one would expect to see something like, "if you ever get down to California...", since California is South of Oregon.
If talking to someone from Nevada, a state bordering California, the invitation probably wouldn't include "get out to." Instead it might be, "If you ever get over to California."
Depending on the proximity or lack thereof, the "get to California" part would be expressed variously to reflect the perceived spatial relationship.