I believe the only way to add the name online is via the comments section (is it still there?). Otherwise, you will need to call reservations or the property and ask them to add the name to the rez. When the other person arrives, they should ask for your rez and mention up front that you called to add their name to be able to check in first.

Since you don't plan on being there, she may not get any status benefits and technically, you should not get points. If there is any chance she will be staying at Marriott properties more often (even with you present), sign her up for her own account. More and more properties are catching on (if they haven't already) to the games being played and even spouses are not getting credit when the other pays for the room! T&Cs state member must stay and pay to get credit and since cards are typically issued in the person's name, if they don't match the property is not required to issue stay/point credit.