with open('group1', 'r') as file1:
    with open('group2', 'r') as file2:
        same = set(file1).difference(file2)

same.discard('n')

with open('some_output_file.txt', 'w') as file_out:
    for line in same:
        file_out.write(line)

group1:
test:x:1234:mike,john,scott
test2:x:1234:mike,john
test3:x:1234:tim,dustin,Alex

group2:
test:x:1234:mike,scott,john
test2:x:1234:mike,john,scott
test3:x:1234:dustin,tim

missing group1:
test2:scott

missing group2:
test3:Alex