Originally Posted by I

It's sad, isn't it? All these older guys partying way past their
prime."


I don't understand the word partying in this sentence
Is it comes from "are partying" or "who are partying"?
If it the present continuous, why doesn't "are" or "who are" is written before it?

Originally Posted by TDOL

We can omit the 'who are'. Sometimes, we use the present or past participle as an adjective and omit the relative pronoun:

John, interested in stamps,...
Here, we can skip the 'whois'.