There're a few ways to get it done. Check Re: Count Unique Customers in Footer Thread #1102497 for some ideas.

>I have a report that looks like this:
>Group Header 1: Tutor
> Group Header 2: Class
> Group Header 3: Student
> Detail: Sign-ins
> Group Footer 3: Student
> Group Footer 2: Class
>Group Footer 3: Tutor
>
>I want to count the number of students (appearing in group header 3) per tutor. I tried adding a field with expression = student, Calculate = Count, Reset based on Group = Tutor and put that field in the Group Header 3 band. However, I'm getting a count of the number of Sign-ins and not the number of students. I also tried putting a field with expression (Select count (student) where tutor = reportcursor.tutor), but that just generated an error. Any ideas?
>By the way, I'm trying to solve this in the Report Designer itself as the application is already distributed and I am trying to help a user customize the report with this field (users can edit reports via the application). TIA.