Timestamp fields record the date and time in seconds so it is unlikely that you will be able to group by this field as every field has a different time as well as the date.
If you don't need the time, you could change the field to date and have the date auto entered
You can add a separate date field like you describe
You can define a calculation field that extracts the date from your existing timestamp field and sort by that field instead of the timestamp.
GetAsDate ( TimeStampField ) could be used in that calculation field. (Select Date as the return type.)
Many thanks for your help on these forums. I did as you suggested above ... and oddly enough, my calculated date field is showing up in this format:
734003, 734200, etc.
Where as my timestamp field has: 8/15/2010 8:49 AM
and my "GetAsDate (Timestamp) formula yieds this response: 733999
Doh - scratch that, I revised the calculation, saw the option to display as "number" and changed to Date.