There are summary functions (sum(), count(), distinctcount(), etc.) that are available in Crystal. However, none of them push the summary down to the database - they do the calculation in memory. In your case, I would do the following:
1. Create a group in your report on either IDVolunteer or a formula like this:
{volunteers.lastname} + ", " + {volunteers.firstname}
The group will automatically be ordered by whatever you group on.
2. Add the fields that you want to see for each volunteer to the group header section. Suppress the Details section.
3. Add a Count summary - to do this either go to the Insert menu, and select Summary or click on the summary button which has a Greek Sigma that looks like a capital "E" on it. The wizard is pretty self-explanatory. Select the field or formula to summarize, set the summary type to Count, then set the summary location to the group you created in step 1.
4. The count will appear in the group footer section. Move it to the group header and then suppress the group footer.
This should get you what you're looking for.
-Dell