If you need to assist a user, but don't want them to see your user account when they log in, learn how to hide a user account on the macOS login window.
1. Log in as an admin user.
2. Use this Terminal command. Substitute the short name of the user that you want to hide for “hiddenuser”:
sudo dscl . create /Users/hiddenuser IsHidden 1
The user account is also hidden in System Preferences the next time it's opened. This command can't be used with the Guest user account. Learn how to manage the Guest user account.