all together; unanimously; in a body
Describes a group acting as one, with everyone participating. Often used when people do something collectively or share the same opinion.
The villagers all turned out together to take part in the festival.
The whole family went on a picnic together.
The residents unanimously opposed the plan.
The people unanimously supported the decision.
総出 emphasizes that every single member of a group turns out (e.g. for work or a search), while こぞって more broadly describes any unanimous group action.
みんな simply means 'everyone' or 'all', while こぞって emphasizes the collective, unified action of a group.
Derived from the verb 挙る (こぞる, 'to gather together'), which is now archaic. The adverbial form こぞって became the standard modern usage.