Using the Gerrit web interface, is it possible to add multiple reviewers at once to a given commit? Currently, it appears that reviewers have to be added one by one: trying to add a list as input fails regardless of how it is separated.
No, it isn't possible to add multiple reviewers at once using Gerrit UI. You have to add one by one or use REST like in this example:
curl --request POST -d@- --header Content-Type:application/json https://GERRIT-SERVER/a/changes/CHANGE-NUMBER/reviewers << EOF
{
"reviewer": "USERNAME1",
"reviewer": "USERNAME2"
}
EOF