Translation:I am going to the mountain, are you coming too?
So basically, if there is no implied movement (from inside, towards outside, up, out, down, etc ), "to" is hoz/hez ?
-hoz/-hez is "toward" or "to the general area of" or "to a position near." It indicates that the destination is someplace generally at or near the thing (but not specifically ON it or IN it).
He's going to the direction of the mountain. Where they can start going up the mountain or where they want to go.
Én felmegyek/mászok a hegyre. - I go up the mountain.
Én lemegyek/leereszkedek a hegyről. - I go down from the mountain.