I'm afraid your roommate is right, at least for the start of the fighting. According to Herodotus, our main source for the battle, these were the numbers of Greeks who fought (7.202-3):

three hundred Spartan armed men; one thousand from Tegea and Mantinea, half from each place; one hundred and twenty from Orchomenus in Arcadia and one thousand from the rest of Arcadia; that many Arcadians, four hundred from Corinth, two hundred from Phlius, and eighty Mycenaeans. These were the Peloponnesians present; from Boeotia there were seven hundred Thespians and four hundred Thebans. In addition, the Opuntian Locrians in full force and one thousand Phocians came at the summons.

They all fought together on the first few days of the battle, each with their own general but under the overall command of Leonidas of Sparta. However, once the Persians discovered they could surround the Greeks by using a goat-path, lots of the allies grew fearful and decided to leave (Hdt. 7.219-22). Herodotus suggests that Leonidas might have purposefully sent the other Greeks away, to secure the glorious reputation of Sparta and himself. In any case, only the Thespians and Thebans remained with the Spartans for the final stand, and it is this part of the battle that became legendary. The Spartans themselves erected monuments to commemorate their fallen, as well as the other Greeks, and the bravery of the 300 became part of the 'Spartan Mirage' - the popular image of Spartans as invincible fighters.

For sources, you should definitely read Herodotus - mostly because it's just fun. In terms of modern scholarship, Paul Cartledge is one of the foremost experts on Sparta, and has a book specifically on Thermopylae.