At the start of each month I retrieve a list of 100+ names of people
currently subscribing to a group. I compare this list to the one
retrieved last month. I can easily count the number in the list and the
number increase or decrease. I'd like to calculate the following:

1) New - The number of people not on last month but new to this month's
list.
2) Left List - The number of people on last month's list but not on
this month's list.
3) On Both - The number of people on both lists.

I assume these would be some sort of array formulas but I'm not quite
sure how to get started.

Thanks for the help.

- John