How can I make it possible for other users to see each other?

I’m not able to get other users. Does user Instance need permissions? If yes, then how can I make it possible for other users to see each other?

Yes, all data is private by default.
To allow other users to see other users right, when a user registers i.e. when you call createAccountand the user logs in the first time, create permission which gives all users access to Query.find or Query.get depending on which function you’re using.