Yes, you can use 사람 in some cases, but there are also others which may be more common.
The word you use depends on who they are to you. Here are some possibilities.
- for your (young) friend, and any children => 얘(이 애/아이), 쟤(저 애), 걔(그 애).
- neutral reference for adult => 이/저/그 사람 (male, female), 이/저/그 남자(male), 이/저/그 여자(female).
- respectful reference for adult => 이분, 저분, 그분(male, female), 이/저/그 남자분 (male), 이/저/그 여자분(female).
(Use of 얘, 쟤, etc. declines with age, and 분 is standard when the person is present. Openly referring to someone as 사람 in their presence may be considered rude.)
e.g. 쟤 왜 저러니? / 저 사람 왜 저래? (when you're away from the person) / 저 분 왜 저러셔?
e.g. 얘가 바로 내가 전에 말했던 그 친구야 / 이분이 바로 제가 전에 말씀드렸던 그분이에요.
그, 그녀, 그들 are mostly used in writing(especially novels) and sometimes in drama narration and the like. Korean language books and sites might use them because it's too hard to teach what's really used in real life, but it's very rare to hear these words in real conversation (one way to verify this is to see if you can find their use in realistic drama or movie scenes).