The Scheduling tab of a meeting that you've been invited to. You can use this to see who else is coming to the meeting. (If you are using Microsoft Exchange Server for your e-mail, you'll see the colored bars which indicate free/busy time for attendees.)
Want to know who else is invited? It may be considered rude to ask, but with Outlook you can look for yourself and no one will be the wiser. Open the meeting request and click the Scheduling tab. You'll see a list of all of the other people who were invited to the meeting. Note, however, that only the meeting organizer knows who is coming for sure. Why? Because only the meeting organizer receives the responses to the meeting request, and it's the responses that report back to the meeting organizer about who is coming.