Postgres 10. Hide users/roles from other users
pgAdmin is using pg_roles to show that information, so it is enough to run

REVOKE SELECT ON pg_catalog.pg_roles FROM public;


But since the information is available in other views as well, you would need to hide them, for instance the pg_user and pg_shadow views.

It should not stop users from logging in nor breaking anything since those views are not used during authentication, so you should make sure that you didn't change anything else. 
The logs might show you the reason for why they can't log in.