Meaning of fall in

(verb) break down, literally or metaphorically; "The wall collapsed"; "The business collapsed"; "The dam broke"; "The roof collapsed"; "The wall gave in"; "The roof finally gave under the weight of the ice"

to take one's place in a military formation or line; "Troops fall in!"

become part of; become a member of a group or organization; "He joined the Communist Party as a young man"