Cats do NOT like being stared at. It's a form of hostility to them, and you may or may not have noticed that your cats eyes rarely blink when a stranger's around. But near you, when they're being affectionate, they'll blink. It's a sign of trust, and, to an extent, affection. Thus, to your visitors, merely emphasize that they ignore the cat and not look at it. Given time, and enough of their presence/visits, your cat may warm up to them. Giving treats is difficult if the cat runs under the bed, but if it manages to stay in the room long enough for your friend to rattle a treat bag to pique its furry interest, why not?
Anyway your cat is your cat. It is not anyone else's. Your cat operates on this belief, and so should you. Trying to pick it up, for example, and take it to a stranger will only bring a further fear of strangers despite your proximity's assumed safety granting properties. So to stop rambling here's my tips from what I noticed of my own skittish cat:
- New guests, or anyone in particular, should take care not to stomp as they walk around. Upon entering your place, your cat will be keenly aware of disturbances in footstep vibrations and of course voice. So tell your buddies to tone it down a lil when they come in.
- Do not stare at the cat. It is hostile. This is why cats go up to people who hate cats—because they were ignoring them! Ironic.
- If your cat consents to give someone a chance, emphasize that they only offer their hand to the cat. Your cat will inspect, and if it approves, scent mark it, and allow some form of petting.
Can't think of anything else at the moment. Good luck!