In the Getting Help section, “join the e-group" link leads the old e-groups portal.
You way wan to update it with: https://groups-portal.web.cern.ch/group/atlas-us-chicago-tier3-users/details or https://groups-portal.web.cern.ch/ with an instruction to search for "atlas-us-chicago-tier3”.
In the Getting Help section, “join the e-group" link leads the old e-groups portal.