“If your best bud or girlfriend moves in with you, and they share expenses, is it income to you?”
Yes. No different than renting out space to a stranger.
Nope. My wife is an accountant. She says that it was a gray area that the IRS almost always overlooked back 30 years ago, (and in the case of a lease as long as both parties were on it, the law didn't apply) but now, with "domestic partner" legislation, it's a moot point, even with only one person on the property deed and mortgage (although obviously only the person on the mortgage can claim the deduction.)