I have a user who has scheduled a meeting for every Thursday for the next few months. Each meeting has a different group of attendees so the meeting requests sent out are directed to different people for each weekly meeting.
The user went in to update a specific meeting and noticed that people that weren't sent an invite are now attendees of the meeting. She then looked at other instances of this meeting (other Thusdays) and noticed the same thing was happening on other instances of these meetings, not the same people on every one though. She verified that the meeting hadn't been forward to anyone else via an original attendee.
Here's how she created these meetings just to get further info. She created the meeting as a series, because it occurrs every thursday at the same time of day. But when she invited people she did the invites for each occurrance of the meeting. So each occurrance has a different group of people invited. But also has some other people as attendees that she did not invite to the meeting. Again this is happening on other meetings in this series as well but with different people each time.
We are using Exchange 2010 with Outlook 2010 in a single domain and single exchange server.