To make answering easier my worksheets look somthing like this:

First tab contains:
Group Entered Entry_Date Leave_DATE
TEST NOV 11/1/2007 3/6/2008
CONTROL NOV 11/1/2007 4/5/2008
TEST DEC 12/1/2007 5/10/2008
CONTROL DEC 12/1/2007 12/1/2007
TEST JAN 1/1/2008 1/6/2008
CONTROL JAN 1/1/2008 3/6/2008
TEST FEB 2/1/2008 11/15/2008
CONTROL FEB 2/2/2008 3/6/2008
TEST MARCH 3/1/2008 4/28/2008


Second tab contains:
Sent
Mailing1 11/1/2007
Mailing2 11/28/2007
Mailing3 12/15/2007
Mailing4 1/4/2008
Mailing5 2/5/2008
Mailing6 2/16/2008
Mailing7 3/7/2008
Mailing8 3/18/2008
Mailing9 4/9/2008
Mailing10 5/1/2008