How can I add Google MarkerClusterer library to Smart Map?
UPDATE
It looks like the underlying MarkerClusterer library has been moved (and theoretically improved).
The new library can be found here...

https://github.com/googlemaps/js-markerclustererplus

Not sure how much that affects the original JavaScript example below, but it's probably still a reasonable starting point.

ORIGINAL
As Jalen mentioned in his answer, this functionality isn't (currently) built into Smart Map. However, it's very easy to add yourself...
First, you'll need to add the MarkerClusterer library to your map page. You can link to it normally, just as you would with any other JS library.
Once you've got the library installed, you'll be able to cluster your markers with code as simple as this:
var map = smartMap.map['smartmap-mapcanvas-1'];
var markers = smartMap.marker;

var markerCluster = new MarkerClusterer(map, markers);

Pretty much everything else happens automagically!