(Edited to make it clearer after my understanding's evolved)
How can the map display all the users from all groups? This would be really useful for my use case.
And what is the format for geocoding in a place like the United States? Does Los Angeles, California work? Does Los Angeles, CA work too? Or should it be Los Angeles, United States, or Los Angeles, California, United States?
I decided to only show users from the group someone belongs to, for privacy reasons. It could be enhanced maybe by asking user on their profile if they want their profile to be public. But curently it's not implemented.
Geocoding is done using openstreetmap, so the doc here applies : https://wiki.openstreetmap.org/wiki/Nominatim
Looks like it works with both : https://nominatim.openstreetmap.org/search.php?q=Los+Angeles%2C+ca+&polygon_geojson=1&viewbox=