Uh, usually when such raids are conducted, it's not a "Knock, knock -- oh, pardon, we're looking for X, may we come in?" situation. Usually they bash down the door, toss in the smokers or flash-bangs, and charge in. Then they cuff everyone (at gunpoint) and ask questions much later.
You don't GET an opportunity to invite them in. Never mind the fact that letting any law enforcement officer in w/o a warrant is a bad idea, because more often than not, they'll FIND something to bust you for.