That's the beauty of celebrity endorsements. Just by being on the app, their followers will find them, then word of mouth takes over about spreading the news to that celebrity's followers that there's a new way to interact with them.
Celebrity's - they all know each other, monitor what the other is doing on social media and so on. These days that kind of monitoring is just part of the whole public relations process.
Can you imagine one celebrity who has a game installed, which is up and running, meeting up with another celebrity friend - then they casually ask them - hey I see you've got a game running on that new app, are you making much money out of it. Celebrity number one replies, sure I'm making $XXXX out of it, a week, a month etc etc etc.
Celebrity number two then turns round to his public relations person who follows him everywhere and says, we need to be on that app as well. Look into it asap please!
Result.
Don't forget that's exactly how it will work. The other way is celebrity's will actually tell their other celebrity friends to get the app themselves as I'm making $XXXX out of it.