I am wondering if I can mark the sub-group in its entirety (instead of one by one, individually) as Prospect.

Yes, you can. Example -

1. Click on Lookup-->first Name-->type J ( or any letter) -->click on Group--> if there are records, it will show else, it will say no valid record. Let us assume that you got 3 records as a result of the query ( it can be any query)


2. Click on Edit-->Global Replace-->one screen will come - In the "Field to replace" list, select "Prospect"


3. On the right hand side - select "Value Replace"


4. One input box will appear below - type T ( T for True and F for False)


5. Now click on Replace - it will ask your permission - say yes and then close the small global replace form. Now check - all the records of your sub-queried result have become ticked automatically as "Prospect"