I had added my user to the group lp, but still got this error when trying to list printers.

$ lpinfo -v
lpinfo: Forbidden

I found that the key was in this post on the Arch Linux Forums.

sudo groupadd printadmin
sudo usermod -a -G printadmin $USER

Then I had to edit /etc/cups/cups-files.conf and change the following.

SystemGroup sys root

to

SystemGroup printadmin root

Once I did that and rebooted, I was able to see output from lpinfo -v without needing root access.

$ lpinfo -v
network http
network beh
network lpd
network socket
network ipp
network ipps
network https
direct hp:/usb/Photosmart_D110_series?serial=CN0CAG436D05N9
direct usb://HP/Photosmart%20D110%20series?serial=CN0CAG436D05N9&interface=1
direct hpfax

At this point, I could use the System -> Administration -> Print Settings in MATE to add my printer.

Done!