Strongly based on hexomino's answer, I think you are a
With two, I’m active
on (in the state of being active, functioning or operating)
With three I’m first or second
one (according to zero-based numbering, the element in position one is the second element rather than one-based numbering, where the element in position one is the first element)
With four, I set an example
hone (a sharpening stone used for removing the burr from the blade of a razor, thus setting an example that all razors should follow if you want to cut more accurately)
With all five, I work backstage
phone (a phone has most ─ if not all ─ of its processing inside, that is, in its "backstage").
The usage of the word backstage may not imply its absolute literal meaning.
Backstage is not used literally here, since backstage means situated in the area behind a stage.
I may do good if you use me, but bad if I use you.
I think it's something related to the idea of this article.