It's a long "one-liner" (broken up for readability), but this I think meets the requirements:

sort <<EOT | tr ',' ' ' | uniq -c | awk '{
  if ( tag != $2 ) {
    if ( tot > 0 ) { print tot, tag, countries }
    tot=$1; tag=$2; countries=$3;
    next;
  }
  tot += $1;
  countries = countries "," $3;
}
END { print tot, tag, countries }'
abcd,US
abee,UK
abcd,US
adee,US
adee,UK
EOT
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