You pay $99 for an App Hub registration, then sign in to the device registration app on the desktop with the device plugged in. After that, it's pretty much automatic. I haven't done that much WP dev yet, so I'm not sure about friend's devices, but so far it's been relatively painless.

(Well, except Windows making installing VS and the Zune software almost impossible, but that's just what happens with Windows. On a clean install. After installing all the updates it wanted. Of which half failed the first time. And rebooting. And rebooting again. But none of that is related to Windows Phone, except that as Apple doesn't make tools for their competitors OSes, Microsoft doesn't either.)