This is a natural question, and I hope Apple makes this procedure more obvious in the next iCal:
In a calendar that you administer, click on an event.
Look to the right of the main iCal window; if you don't see a separate information panel, like a drawer, sticking out to the right, click on the button with "i" in the circle at the bottom right of the main iCal window. There's rarely any need ever to close this Info.
Now, in the info panel, click the grey word "None" next to the black "attendees." Start typing the name of someone in your address book. When their name is filled in automatically, hit the
Et voilà! You have just emailed an invitation to someone, and you (and Apple Mail and iCal) stand awaiting their reply. Look in your email, and also in the Notifications panel of iCal (there's a button bottom left of iCal, and "Show Notifications" is in the View menu.