WordNet (r) 3.0 (2006):

fall in
    v 1: 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" [syn: collapse, fall in,
         cave in, give, give way, break, founder]
    2: to take one's place in a military formation or line; "Troops
       fall in!"
    3: become part of; become a member of a group or organization;
       "He joined the Communist Party as a young man" [syn: join,
       fall in, get together]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "fall in":
   break down, cave, cave in, collapse, come together, crash,
   crystallize, deflate, draw up, fall, fall dead, fall down,
   fall flat, fall into line, fall into place, fall into rank,
   fall short, fall stillborn, fall through, fold, fold up, form,
   form a line, founder, gather around, get in formation, get in line,
   go to smash, implode, line up, place itself, puncture, queue,
   queue up, rally round, range itself, shape up, sway, take form,
   take its place, take order, take rank, take shape, topple,
   topple down, topple over, totter