たち is a pluralizing suffix. So, 猫たち means cats, 子供たち means children and 人たち means people.
Why can 友達 mean 1 friend, even though there is the word 友 meaning a friend?
(I see that 達 in 友達 is not written using kana, unlike たち in, say, 猫たち. But originally the suffix used to be written as 達 even in words like 子供たち, didn't it?)