How to select a max value within a group

I'm creating a crystal report that uses two fields (Lease#, ID#), and I want to group records by Lease#, and then only display those records with the maximum ID#.

Lease    ID#

LeaseA   1

LeaseA   2

LeaseB  1

LeaseB  2

LeaseB  3

Using this example, I want these records to be returned:

LeaseA   2

LeaseB   3

I'm sure this is simple but I'm stumped.