The ending is deliberately left open. I think the most popular interpretation is that the three of them faked it all themselves so they could disappear and make people think they were 'taken' by the Blair Witch, hence copying the Blair Witch legend. After all, their bodies weren't found (at least I don't think so. I don't know anything about that Blair Witch sequel or anything else that might have been written). You could also say that the two guys planned it to kill the girl, or that the guy who goes missing killed them both. But the other guy is standing in the corner so he must also have been in on it.
Whatever, you're obviously not supposed to know what REALLY happened because that's not part of the story. I can't believe some people in America actually thought this was real at some point. It's fairly interesting to watch first time, but it would have been better if more happened in the movie rather than an hour of them just walking around wooRAB lost and shouting at each other.