들 : 체언이나 부사어, 연결 어미 ‘-어’, ‘-게’, ‘-지’, ‘-고’ 또는 문장의 끝에 붙어, 그 문장의 주어가 둘 이상임을 나타내는 보조사. 들 is a particle used after a noun, adverb, a connecting particle ‘-어’, ‘-게’, ‘-지’, ‘-고’ or the end of a sentence. It denotes that the subject is plural. ex) 어서 먹세들. 그대로들 있어. 먼저 가고들 있어. 그만 떠들고 어서 잠들 자거라. 외계인이라고들 하죠 = 외계인이라고 하죠들 = 사람들이 외계인이라고 하죠 They say that I am an alien * It is somewhat similar to the usage of "-nya" in Indonesian.

I think the first answer fully explained your question. and so on, '~라고들 한다.' is a expression 'many people say ~' or 'People say ~'

-들 basically marks the plural form. It may seem strange to use it after -라고, but in that case it just means "they". 외계인이라고들 하죠 = They call me an alien