Why would the be at all friendly to humanity? If they're our peers technologically, then we're competition. The main reason to travel to another solar system would be resources, and we're also consuming resources. We aren't friendly with OURSELVES, and the one inevitable truth in evolution is that competition for resources is the core of the entire system. The only way they could be friendly with us is if they're entirely passive and uncompetitive, and a creature like that couldn't compete well enough to survive to begin with.
Why would they be in any way similar to us? We share about 97% of our DNA with a chimpanzee. We share about 60% of the same DNA as a chicken or fruit fly. We would share even less with an alien species. Even assuming that they'd be humanoid is baseless; ours is not an ideal shape for most things, as can be seen by the way hominids are relatively unique in this respect on the planet. About all you can expect for a technological sentient race is that they'd have some manipulator limbs, and some sensory ability. It's just as likely that they'd be ten-armed aquatic critters more similar to an octopus than a human, and who communicate primarily through variations in skin chromatophores. And even there I'm probably being too earth-centric in my predictions.
Open to communication? See above. We're primarily light and sound oriented. We speak, and use visual communication like writing. This is why we immediately went for observing the universe via the electromagnetic spectrum. This is also how things like SETI spend basically all their time trying to discover alien communication signals, in radio waves and the like. A race that evolved underground, however, may not have any visual orientation at all, and thus not use any kind of wave of beam communication whatsoever. The idea that mathematics is "the universal language" is a nice one, but we first need to recognize what we're seeing as a signal to begin with. That's a huge assumption. Some creatures on the Earth use magnetic or electromagnetic senses, or scent-based communication, or touch, etc.
People generally think "alien" means "little guy who looks like a hairless human kid". That's ridiculously unlikely. If we WERE to encounter a creature that looked like that, it would be more likely to be related to us somehow than to be truly alien. Sci-fi writers tend to shortchange the possibilities here, because it's hard to tell a story involving entities that are beyond easy understanding; unless your entire story is about that very difficulty, they tend to make them fairly human-ish just to simplify telling the story they want to tell. Even Star Wars is WAY too consistent and earth-centric in how they designed their "aliens".
---------- Post added 2013-02-03 at 06:05 PM ----------
They also wouldn't bother hiding while they did so, though, for the same reason we don't try to hide from monkeys in the rain forest we're clearcutting.
The point is, at no point is there likely to be a beneficial contact for humanity. And most sci-fi movies that deal with alien invasions don't do the aliens justice; they have them land on Earth to duke it out with us for some ridiculous reason.
Probably one of the most realistic possibilities would be the Predator movie. No interest in communicating with us, at all. Vastly superior technology. They do their own thing and ignore us where possible.