To make a heatmap, simply select latitudes and longitudes using the Map chart type, and then navigate to the "Chart Format" tab. Under "Map Display Settings" on the right, check the "Heatmap" radio button:
This will render the map as a heatmap instead of individual points:
Weights can be specified in a third column by select the column as "weight" in the "Series Settings" tab, and will be included in the Heatmap weight calculations.
For example, to get more total points on the map, let's round latitude and longitude to 3 decimal places:
This diffuses the heatmap a bit, as more total users are included in the map: