How to display a Map with markers and user locations

Use Map class and Map.addItem(label, onclick, style) and Map.addUsers()

On Android, if there is too many markers at the same area for a gven zoom level, the map will display them clustered with a number inside a round circle
function viewMap() {
    Map.addItem("Paris", "48.858238, 2.347918", "viewCity()");
    Map.addItem("London", "51.522020, -0.122198", "viewCity()");
    Map.addItem("Berlin", "52.518198, 13.380823", "viewCity()");