Is there any way for newbies to build reputation besides spending months to become a regular? Maybe some kind of a guarantee given by an already reputable user?
Like someone already said, you can’t skip the process.
In life, we have growth phases, where you grow from an infant to young adult and an adult eventually. None of these processes could be skipped and that’s the same thing that happens here. It’s a building of one’s online presence by going through the forum’s stipulated growth phases.
Reputation isn’t something bought and sold but simply how someone perceives you from your deeds. Should you come up with a way to set yourself up to be seen as reputable, that becomes a scheme and you don’t need schemes to acquire trust. It only means, you’re after something and that might not be good.
It’s always best to let time take its course as the reputable users here have done. Even then, you wouldn’t get the trust of all users.