Instead of doing a formula, you can insert a summary. Go to the Insert menu and select Summary. This will bring up the Summary expert. Select the field you want to summarize, the type of summary (Count), the Summary Location (the group you want to do the count for), and whether or not to create the summary for all groups.
Click on OK and the Summary will appear in either the group footer or header, at which point you can place it where you want it.
-Dell