How can I hide an existing user - El Cap I found commands about how to create a user that is hidden, 
sudo dscl . create /Users/USERNAME IsHidden 1

but how can I hide an existing user?
The command sudo dscl . create /Users/username IsHidden 1 doesn't really create a full-fledged user. The user misses an UID and a PGID and some more attributes.

Instead it creates a "rump user" with a new Native Attribut Type: IsHidden with the value 1.

You can use the very same command for any existing user and add the IsHidden attribute with the values 0|1 though and after a restart the user account will be hidden in the login screen as well as in fast user switching (and in the System Preferences -> Users &amp; Groups) if the value is set to 1.

Users with an UID ≤ 500 are hidden by default!