You cannot modify a reservation online, not even to add additional guests. You either have to call MS or send them email, listing the info for the additional guests.
To save time,I usually just book myself on a reservation at 11 or 7 months, then add friends later. Otherwise you have to type in the names and addresses and typically at 8:00am I am on the way out the door for work. I use the "contact us" on the member website to add later.
I just did this yesterday and just did the online chat. Took about 3 minutes total. However, I have done it this way before and it has taken longer. I have also emailed and just ask them to add a name.