Use a running total field. Insert, field object, running total field, make it a count running total, evaluate on every record, reset on change of group.
If you are on version 6 or older of CR running totals are not available and you will need to use the three formula technique with variables, see teh FAQ area for a very good FAQ on how to do this. Software Support for Macola, Crystal Reports and Goldmine
dgilsdorf@mchsi.com