'들' is more broadly used than a few set phrases when a plural subject is elided and you want to imply the subject is plural. It is correct to assume that it is more used in speech than writing, but it is not informal.
'들' has mainly two functions:
- Noun suffix (접미사): It makes a countable noun (or pronoun) plural. For example:
너희들 (plural of 너희) 여기서 뭐하니? What are you doing here?
서울에는 사람들 (plural of 사람)이 많다. There are a lot of people in Seoul.
- Auxiliary particle (보조사): It is placed after a verb, adverb to imply an elided subject is plural. For example:
시간이 충분하니까 천천히들 해도 돼요. Since we have enough time, you (plural,
elided) can do it slowly (you can take your time).
공원에 앉아들 있었다. (Elided plural nouns or pronouns) were sitting at the park.
먹게들 (가만) 둬라. Let (elided plural nouns or pronouns) eat. Leave (elided plural nouns or pronouns) alone when they eat.
When it is ised after an adverb, you can try to place the elided plural subject back and see if it makes sense. For example:
어서들 와. -> 여러분들 어서 와.
잘들 했다. -> 여러분들 잘 했다.
앉으세요들. -> 여러분들 앉으세요.
시간이 충분하니깐 천천히들 해도 돼요. -> 시간이 충분하니깐 여러분들 천천히 해도 돼요.