google-group-import.sh

#!/bin/bash

function usage() {
        (
        echo "usage: $0 <auth-token> <group-address> <mbox-dir>"
        echo "To generate an auth token go to https://developers.google.com/oauthplayground/ and get an access token for Google Groups migration"
        ) >&2
        exit 5
}

AUTH_TOKEN="$1"
shift
GROUP="$1"
shift
MBOX_DIR="$1"
shift

[ -z "$AUTH_TOKEN" -o -z "$GROUP" -o -z "$MBOX_DIR" ] && usage

for file in $MBOX_DIR/*; do
        curl -H"Authorization: Bearer $AUTH_TOKEN" -H'Content-Type: message/rfc822' -X POST \
                "https://www.googleapis.com/upload/groups/v1/groups/$GROUP/archive?uploadType=media" --data-binary @${file} || exit 1
done

I've found https://github.com/XaviTorello/mail2googlegroup

Recommend changing --data-binary @${file} to --data-binary @"${file}" to fix issues with files having spaces.