Hi Mike,
Your activities must be assigned to a user on your account. As a work around, you can either add each of these employees as inactive users to your account or add 1 inactive user to just assign all of these activities to.
An inactive user will not count against your licenses so this will not increase the cost of your subscription. Just make sure that when you add the user, the User can sign-in option is unchecked.
You can also disable the user from appearing in any of your lists in Method by unchecking the Login is active box in the edit user settings.
I hope this helps.
-Audisho