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.
See also: 揃う
The whole family went on a trip together.
They were all of the same opinion.
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.
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.