I'm in France and was able to sign up. You just need access to a US phone number for a few minutes, and there are many ways you can get one. One is to signup for 3jam for one month, another is to get a SkypeIn number. It's probably possible with Twilio as well.

That having been said, you don't get as much use out of it as if you lived in the US. But I use it for free unlimited SMS (incoming and outgoing) to my friends in the US, and for voicemail with an American number .