"Hear ye, hear ye. Come one, come all, and happy Fourth of July."

Does "Come one, come all" mean "Come, everybody"?

Yes that's right.

"Come one come all" is a old English idiom which was a guarantee. It would be included in a posting for some event, maybe a play at some location. 300 years ago travel was a real effort and this was a guarantee that if you were the only one to came to the event it would go on and your travel effort would not be wasted by a cancellation of the event. Come one come all, the play will go on.