Hopefully, the following references can get you started.  First, you should
create a query to calculate the number of participants by agency.  This
normally done with a "GROUP BY" query, using the "Count" aggregate function.
The following KB article give more information on creating such a query.  In
the article, the instructions refer to the SUM function.  Just use the Count
function instead.

http://support.microsoft.com/default.aspx?scid=kb;en-us;304361

Your query might look similar to the following:

SELECT Agency, Count(*) as AgencyCount
FROM MyTable
GROUP BY Agency

Once you have created the query, you can use it as the basis for your report
(Record Source property).  Below is a download link for some sample Access
reports that you can use as a guide.

http://www.microsoft.com/downloads/details.aspx?familyid=71DB24A5-C76F-4EDA-BC9A
-5FAC336EF7F1&displaylang=en

I am new to Access and have created a conference database.  I need to report
on the number of participants who attended by their agency.  How do I set up
a report to reflect this information?