Out there, out there, my prefix you cannot find
Opposite of out there: in or inter
An endless line, a stanza, is what my infix is
Originally thought verse, but comp can be short for composition and can be musically oriented.
Lost one in many, and my suffix I become
Going off of @Walt's idea, many - m = any
The word am I, as you are seeing here!
While there are many definitions, they're all similar to: a number of individuals assembled or associated together; group of people.
Well, that's what we have here on SE.