Before to read further answer the question,

How would you say in Russian, “All people were there”?

I am asking that because I’ve heard a lot of times when people (not Russian native speakers) were saying, “Всё люди” and that was wrong.

That’s why I would like to remind you one more word:

всё – sounds like /fsYO/ – everything

весь / вся / всё / все  – all (masculine / feminine / neuter / plural)

Например:

Я спал весь день – I slept all day (because “day” is a masculine noun)
У тебя вся машина грязная – Your car is all dirty
Это всё на сегодня – that’s it for today
Все мои друзья работают сегодня – all my friends work today

The right answer is

“Все люди были там” – all people were there , sounds like /fsYE/