Yes. If someone outside your domain sends you an invitation from another type of calendar, you can typically accept the invitation from Google Calendar. If possible, Google Calendar inserts an Add to Calendar link in the invitation. Simply click the link, and the invitation is added to your Google calendar. Otherwise, if the invitation is in the standard .ics format (such as Microsoft Outlook and Lotus Notes invitations), you can download the .ics file and upload it to Google Calendar.