You don’t have to set them up to do this. This happens automatically as a function of Outlook. In fact when the meeting request arrives in their inbox the “sniffer” service automatically adds it to their calendar as “tentative” unless they delete it, then it will come off the calendar. Accepting the invitation changes it from “tentative” to confirmed automatically.
<a href=”http://office.microsoft.com/en-us/outlook-help/outlook-meeting-requests-essential-dos-and-donts-HA001127678.aspx?redir=0″>Outlook Meeting Do’s and Don’ts</a>