I think it really depends on how well they were socialized as kittens, their personalities, and whether they feel safe and comfortable around your guests.
Pippin will run when people come over, but only halfway up the stairs, then he will come back down and socialize, sometimes getting into their drinks or jumping on them unexpectedly.
Morris and Simple don't seem to even notice anyone is over, except that it's time to swarm them for lovin'.
Arwen is always hiding, but sometimes comes out if she hears my good friend, Tim.
If your cats usually come running when they hear a sound, like pouce treats rattling in their can, you could have your guest shake it, and give them a treat for visiting. That way they associate being social with a reward.
Sometimes, cats are more likely to come out when there is only one person over. They feel less intimidated by the noise and figure they can avoid one person. As they get used to you and the sounds of your friends and family, they might warm up to them.
You could also try bringing kitty out and putting him out of reach on a cat tree, so he can observe while feeling safe. He might even come down for a visit.
If your friends are so inclined, I recommend getting down on the floor to meet new animals. Your friends can also try getting out some of the kitty toys and playing with them. Cats are curious by nature and may want to come out and see what's going on.
Playing soothing music
Speaking in low, calm tones. Don't make loud noises that might startle them.
Praising and rewarding behavior you want to encourage.
Try to be patient and remember they may NEVER be very social. A lot of it depends on your friends' willingness to get to know the cats on their terms.
Cats are unlikely to remember people that aren't around all the time. Take it slow with each visit.