A somewhat old-fashioned or rustic way to address a group of people, often used in period dramas or by older speakers. Can be friendly or slightly formal depending on context.
Thank you all for gathering here today.
Compound of 皆 (みな, 'all') + の (possessive particle) + 衆 (しゅう, 'people'). The exact historical derivation is uncertain, but it has been used as a collective address since at least the Edo period.