A maximum of one unique user can be connected to each printer, and therefore a printer can not be connected to a new user if it is already connected to another user, unless you log in to the original user and press “Disconnect / Disconnect from user ”. You can find this option by going to "user settings" (the middle of the 3 tabs at the bottom) and pressing the gear in the upper left corner.
This helps ensure that one's neighbor does not steal one's printer and helps increase security.