Translation:How is everyone?
Please say this loudly everytime you get on the bus or train in Japan.
みなさん is the polite form that you would use to or about a group of people with whom you are not friends or otherwise an equal to.
This is a sentence with at least two possible different translations.
a. "How is everyone?"/"Is everyone okay?"
This is where the particle は is implied, and the meaning is just the same as that of みんなさんはお元気ですか. This is probably the most polite translation of the sentence.
b. "Everyone, how are you?"/"Everyone, are you okay?"
Here, the translation is more direct and more literal. You are addressing everyone and checking on them directly, but is not as polite.