I've used the following sql query:
select email from galaxy_user where deleted = 'f';
An example of a bash script I use to parse out some unwanted addresses and output everything to a simple list:
#!/usr/bin/env bash psql -U galaxy -c "select email from galaxy_user where deleted = 'f';" -t | grep -v "igv_display\|email@example.com\|firstname.lastname@example.org"
You could use the users API (as a galaxy admin user, you'll see all users at http://<your_galaxy_server>/api/users ) to fetch this in a programmatic way, but there isn't an easily accessible 'all the users' list built into the interface. Let me know if you need more details to use the API.