The new PS4’s firmware introduces a new feature called Share Play. This function allows players to invite friends to join in their games. Your guest could take the rides of a solo game if you want to let him to play or to beat a hard section. Even, this player can be invited to join you in a multiplayer scenario.
It’s a cool way to test a game if you want to buy it.
In the same vein, your guest won’t have access to the PS4’s menu, but he/she might delete your save files (in game). Take in mind the Share Play it’s like to give the control to other person. Therefore, before to give access to an untrustworthy contact, upload your saves to the cloud. He could troll you nicely.