揃そろって

Meanings

all together; in a body; en masse; all alike

Describes a group doing something together or being in the same state. Often used when everyone participates or shares a characteristic.

一いっ斉せいにN2
揃って means 'together' or 'as a complete group' and often implies physical gathering, whereas 一斉に focuses on the exact simultaneity of the action.
こぞって
揃って describes people or things being together/uniform in action, and is a slightly more common everyday word than the more literary こぞって.
ともにN3
そろって highlights that a group acts or is present all together, close to the 'both' sense of 共に in AもBも共に patterns.
共とも々どもN1
揃って emphasizes that all members are present and acting as one; 共々 is used more in set, polite phrases like 家族共々 or 皆様共々.

The te-form of the verb 揃う (そろう, 'to be complete, to be uniform'), used as an adverbial expression.