Comment by pferde

Each profile in GrapheneOS is encrypted separately, and switching profiles require entering a PIN (plus additional biometric methods if you set them up for that profile) before the data is decrypted and accessible.

So yes, you can hand the phone over to a friend or family, and they cannot get to any other user profile. Or you can set up a separate profile just for them, and they will have their own isolated set of apps - something like a separate user account on a desktop PC. And if only they know the PIN for their profile and you don't, they can keep secrets from you on that profile.