According to John Tvedtnes, among the Nephites, we find that there were clear-cut tribal distinctions. For example, though the sons of the people of Ammon, who fought under Helaman in the great war, called themselves by the more general term of Nephites (see Alma 53:16), they nevertheless appear to have been segregated from the main Nephite army. Helaman noted that his two thousand Ammonite warriors "were descendants of Laman, who was the eldest son of our father Lehi" (Alma 56:3). This implies that none of them was descended from Lemuel or the sons of Ishmael, who also formed the Lamanite league. The fact that they did not mingle with the other Nephties is demonstrated by Helaman, who wrote in Alma 57:6 that "we received a supply of provisions, and also an addition to our army, from the land of Zarahemla, and from the land round about, to the number of six thousand men, besides sixty of the sons of the Ammonites," who had come to join their brethren." That these "brethren" were part of the same tribal affiliation is later confirmed by Helaman when he refers to "my little band of two thousand and sixty" (Alma 57:19-20) [John A. Tvedtnes, "Book of Mormon Tribal Affiliation and Military Castes," in Warfare in the Book of Mormon, F.A.R.M.S., p. 303]
How does this square with the fact that most of the Lamanite converts of Ammon apparently came from the land of Ishmael, "the land being called after the sons of Ishmael, who also became Lamanites" (Alma 17:19)? [Alan C. Miner, Personal Notes]