According to this forum post over at Raspberrypi.org, you need to modify lightdm.conf (/etc/lightdm/lightdm.conf). There is an "autologin-user" parameter in that conf file that you can modify, and, paraphrasing the last poster, several other setting you need to change:
sudo sed -i 's/^greeter-hide-users=true/greeter-hide-users=false/g' /etc/lightdm/lightdm.conf sudo sed -i 's/^\#greeter-allow-guest=true/greeter-allow-guest=false/g' /etc/lightdm/lightdm.conf sudo sed -i 's/^\#greeter-show-manual-login=false/greeter-show-manual-login=true/g' /etc/lightdm/lightdm.conf sudo sed -i 's/^\#allow-guest=true/allow-guest=false/g' /etc/lightdm/lightdm.conf sudo sed -i 's/^\#allow-guest=true/allow-guest=false/g' /etc/lightdm/lightdm.conf sudo sed -i 's/^\#autologin-user-timeout=0/autologin-user-timeout=10/g' /etc/lightdm/lightdm.conf # also comment out default login from 'pi' sudo sed -i 's/^autologin-user=pi/\#autologin-user=pi/g' /etc/lightdm/lightdm.conf
Hope this helps.