Outlook for Mac - do not automatically add meetings to calendar
This actually isn't client-side (that is, Outlook) setting.  This is a server setting on Exchange.  Depending on how your Exchange server is configured, this might or might not be a setting that you are allowed to change.

If you are allowed to change it, you can do so via Outlook Web App.  The instructions that I'm giving here are for Exchange 2013.  


Open OWA.
Click the "Settings" gear (upper right corner), then select "Options".
In the left navigation pane, select "Settings", and then select "calendar" in the main pane.
Scroll down until you see "automatic processing".  If you can change this setting, you'll see a tick box titled "Automatically place new meeting requests on my calendar, marked Tentative".  Untick that box.
Click save, and then exit the settings.


Earlier versions of Exchange also allow this, although the path to it is slightly different depending on which version of Exchange you're in.  They should all be in the calendar settings, you might have to poke around to find the exact location in your version of Exchange.