That is, without doubt, the most boring and uninspiring title that I've ever come up with for a blog entry. Sorry. I just couldn't think of a better way of describing this fairly ordinary, but still rather useful, tip.
As you may know, when inviting people to a meeting you can decide whether they are required attendees, optional attendees or a resource (i.e. not a human being, but a meeting room or other asset like a projector that your company has registered in active directory and made available for booking through Exchange Server). In email parlance, setting someone as 'Required' is the equivalent of sending an email "To" them, while being 'Optional' is the same as being CC'ed. Finally, being labelled a resource means that the meeting requests will be BCC'ed to you.
But you may have noticed that Outlook only presents a To... line when creating a new meeting request. So how do you change someone from Required (i.e. 'To') to Optional (i.e. 'CC')?
There are two ways. The first and most widely used is to press the scheduling button (in Outlook 2007; use the scheduling tab in Outlook 2003) which will present you with a shared calendar view (if available) of all participants' availability. Next to each person's name will be a small icon showing them as a required attendee (up arrow in a red circle), optional attendee (i symbol in a blue circle) or resource (house symbol in a green circle). Right-clicking on these icons allows you to change each invitee's type.
The second way is simpler and quicker. But also less commonly used. Simply hit the To... button in your meeting request (number 1 in the image below) and then enter the names of the people or resources that you wish to attend your meeting in any of the three text boxes (number 2 below). Using this technique makes it much easier to move people from one type to another and gives the added advantage of allowing you to search through your Global Address List or Contacts using the Search options.