To enter in Real-Time Collaboration mode, first, you need to Sign In with your email and password inside Project Plan 365 app with an Editor account. Go to the menu Share – Real-Time Collaboration or click on the Real-Time Collaboration Toggle in the Quick Access Toolbar. If your file is not saved in Drive 365 cloud, you will be asked to save the file in Drive 365 and then the mpp file will be opened in Real-Time Collaboration mode.