The API is similar to that of the Google Maps version ([1]):

var map = new L.Map('map');
var lat = ...;
var lng = ...;
var pos = new L.LatLng(lat, lng);
var zoom = 4;
map.setView(pos, zoom);

var clusterer = new LeafClusterer(map);

var lat = ...;
var lng = ...;
var pos = new L.LatLng(lat, lng);
var marker = new L.Marker(pos);
clusterer.addMarker(marker);

or for batched addition:

var markers=[];
for each marker (m) {
   markers.push(m);
}

clusterer.addMarkers(markers);



[1] http://gmaps-utility-library.googlecode.com/svn/trunk/markerclusterer/