I am working on an XLS which tracks the success of a marketing campaign. My
question is this, given the marketing campaign and source data described
below, how can I track how many mailings were received by my test group w/in
a certain timeframe?

The marketing campaign runs for 6 months. Each month, at different times, 2
mailings are sent out. for example, on 2/1 a mailing was distributed. On 2/15
another mailing was distributed.

At the start of the campaign, 200 people were in the test group. Each month
people leave, or are added to the group.

So, if someone enters the group at the beginning, and leave it midway
through, they will receive 6 mailings. However, if they join in the second
month and stay until the end, they will receive 10.

Source Data:
The source data, that the function needs to work with are the following
columns: TEST_Group, Entry_Date, Leave_Date, and Mailing1 (date), Mailing 2,
etc.

Each row represents a customer. Ultimately, I want to list the number of
mailings received by each customer during their time in the program.

What is the best way to do this in Excel?