This should be the only way to do it in my opinion. You server could check for more people per update then the app could do it self. Also by using push notification you will make sure that you app is not running in the background (this is not even possible for your kind of app) and draining the battery.
A cron job that runs every so often that calls a web page, shell script, etc.. will also do. There are many option and there is no 1 answer.