Kevin,
I am having the same issue (another account from the same organization is already signed in mac outlook) with my 2011 Outlook (Office 360) for Mac. I have the latest Outlook software and have tried removing and reloading the Office and Outlook products.They are the only version of the software loaded on the Mac. I am only using 2 of 5 licenses. I had used the software for two months without any significant issues. This issue arose after a data migration for another Mac (could this be an issue?) Everything else as I have determined up to this point runs as it should. Your help is greatly appreciated.
Steve