Append user to People&Group Field using jQuery

Hi, 

We have an event tracking functionality. I would need to capture the attendees of the event on click of a button. On button click, the current logged in user should be added to the attendees field in the events list. 

I am able to update the value, but on every update it overrides the existing user. I would insted want it to append the current user to the existing users in the Attendees column. Please find my code below,

$().SPServices({
        operation: "UpdateListItems",
        async: false,
        listName: "Upcoming Events",
        batchCmd: "Update",
        ID: 1,
        valuepairs: [["ParticipantsPicker", FullUser]],
        completefunc: function (xData, Status) {
               }
});

Could anyone suggest me an approach to append the value, instead of overwriting.

Raj