Something must have scared him, perhaps when more people come in and talk loudly. My cat hates visitors, too. He runs under my bed and stays there until my guests are gone, then he'll come out and all is normal.

When he was younger, he would stay where he was - mostly, but as soon as he hears the doorbell these days, he's under the bed.

If my catsitter comes in and uses a key, he doesn't hide.

All I can suggest is to ask your guests not to talk or make any other noise for the first 5-10 min., but I can see that might be difficult.

Good luck!