$ sudo pacman -S mate
[sudo] password for matthew: 
:: There are 14 members in group mate:
:: Repository community
   1) caja  2) marco  3) mate-backgrounds  4) mate-control-center  5) mate-desktop  6) mate-icon-theme  7) mate-menus  8) mate-notification-daemon  9) mate-panel  10) mate-polkit  11) mate-session-manager  12) mate-settings-daemon
   13) mate-themes  14) mate-user-guide

Enter a selection (default=all): 
resolving dependencies...
looking for conflicting packages...

Packages (22) exempi-2.5.2-1  libmatekbd-1.24.1-1  libmatemixer-1.24.1-1  libmateweather-1.24.0-1  libxklavier-5.4-3  libxpresent-1.0.0-2  yelp-3.38.1-1  yelp-xsl-3.38.1-1  caja-1.24.0-1  marco-1.24.1-1  mate-backgrounds-1.24.1-1
              mate-control-center-1.24.1-1  mate-desktop-1.24.1-1  mate-icon-theme-1.24.0-1  mate-menus-1.24.1-1  mate-notification-daemon-1.24.1-1  mate-panel-1.24.1-1  mate-polkit-1.24.0-1  mate-session-manager-1.24.1-1
              mate-settings-daemon-1.24.1-1  mate-themes-3.22.21-1  mate-user-guide-1.24.0-1

Total Installed Size:  289.26 MiB

:: Proceed with installation? [Y/n] y
(22/22) checking keys in keyring                                                                                                               [#######################################################################################] 100%
downloading required keys...
error: key "3B94A80E50A477C7" could not be looked up remotely
error: required key missing from keyring
error: failed to commit transaction (unexpected error)
Errors occurred, no packages were upgraded.