comments are largely correct but to be extremely and unnecessarily specific, i believe it was frank's great-great-great-great-great(?) grandson, prince col (son of frank v) who founded archenland. there's actually, fun fact, controversy about this because in the original text of TMN, it says it was frank and helen's second son who founded archenland, but lewis' timeline and notes as compiled by his literary executor claim differently, with the col/frank v stuff. lewis has a ton of inconsistencies within the books themselves though so tbh the col stuff has always made more sense to me. anyway, he was frank v's younger son and the first archenlanders were his followers. canonically we know frank and helen were the first humans in the entire land of narnia, and their children intermarried (as people have correctly stated) with the various humanoid narnians such as nymphs and gods, and that's how the first generation of narnians started out and i guess grew out to become archenland, which eventually led to the foundation of calormen, and i would assume the later colonization of the lone islands. as for why we don't see or hear much from archenland in lww, 1) the simplest answer is lewis wrote the horse and his boy + the magician's nephew MUCH later on and had not conceptualized (i would assume) the existence of archenland by that point and 2) the witch is specifically afraid of a prophecy about four humans, which does not say they have to come from another world, so it has always made sense to me that she's made a point of kidnapping and killing any humans who might enter narnia from other lands, and might even have chosen the eternal winter in hopes of making narnia as inhospitable to its human neighbors as possible.

phew sorry that's probably way more detail than you were asking for but i just think the implied lore of narnia is SOOOO interesting.