members.py

#!/usr/bin/env python3

"""Returns the number of members of a master group in other groups.

Accepts Google Groups member export CSVs. Usage:
    ./members.py < master.csv other.csv groups.csv
"""

import csv
import sys

def membersOf(r):
  reader = csv.reader(r)
  out = set()

  for row in reader:
    if '@' in row[0]:
      out.add(row[0])

  return out

def membersOfFile(f):
  with open(f) as r:
    return membersOf(r)

others = [(x, membersOfFile(x)) for x in sys.argv[1:]]
master = membersOf(sys.stdin)


for name, s in others:
  print("{} has {} members, {} are in master".format(name, len(s), len(s.intersection(master))))