Hello Maureen,
Here's my reply. In order to achieve this, firstly you have to make a copy of your CRM_NewActivity screen. Then when you edit the screen, Go to Hidden Section, under that section you will find a Hidden Function Button.
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Click on edit on the Hidden Function, and go to Step 2 of 3:Button Actions and look for Send Email action in the list. Once you find the Send Email action click on edit.
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Remove the value from Email BCC action result, save and close it, Publish the screen and try to assign an activity and test it. I have test it on my account and it is working fine.
Hope this helps,
-Inder