Hi Aaron,
The Assigned To field in the Outlook Plugin defaults to whatever credentials you used to sign into the plugin. If you are using your email address to sign in, you email address will populate the Assigned to field when adding a new activity/case/opportunity (which can cause issues when saving), but if you signed in using your username, this will auto populate instead.
There isnt a way to set the default value to a different employee than the one currently signed in, but updating the outlook plugin is something we will be looking into for the future, and Ill make sure to add your feedback there!
Thanks Aaron.
-Ben