When you add a user to a new group, you're often told you need to log out and log back in for the new membership to take effect. If you're at the terminal running screen, this is no big deal, but it can be annoying if you're running a lot of graphical stuff that doesn't suspend and resume so easily. Luckily there is newgrp which can adjust your current group ID to match the new group:

$ usermod -a -G groupname username
$ newgrp groupname