onlara selam söyle... say hello to them (from me)

Also, can someone explain to me the difference between onlara and onları?

Yes your translation is correct.

onlar = them

onlara = onlar + a (suffix indicating movement towards) = to them, as in your example sentence

onlarada = onlar = da (suffix indicating in/on/at)

onlarda para yok = they have no money on them

onlardan = onlar + dan (suffix indicating movement from)

onlardan selamlar = greetings from them

onları = onlar + ı (suffix indicating this word is the object of something, often to identify the object that the following verb refers to, and sometimes used to show the word is possessed by something)

onları seviyorum = I love them