When you add an even in iCal, you can invite attendees of the event. Like, I have a phone conference with my kid's teacher Wed at 9:00. I can add my husband as as attendee of the event (as I am adding it to iCal on my Macbook) and then I can send him an invite. When I send the invite my Mac magically sends him an email notification since he is in my address book already. I tried this online on Mobile Me and it did not let me add an attendee or send an invite from there.
From what I tested last night, you can only create a calendar invite from the iCal application that comes with a Mac. I do not think this works using Mobile Me via the web. I will test it again later on for you and let you know if I get it to work.