Except the MapSet…

iex> list = ["apple", "apricot", "banana", "blueberry", "kiwi", "blackberry"]
["apple", "apricot", "banana", "blueberry", "kiwi", "blackberry"]
iex> Enum.group_by(list, &String.first/1)
%{
  "a" => ["apple", "apricot"],
  "b" => ["banana", "blueberry", "blackberry"],
  "k" => ["kiwi"]
} 

With MapSet…

list 
|> Enum.group_by(&String.first/1) 
|> Enum.reduce(%{}, fn {k, v}, acc -> Map.put(acc, k, MapSet.new(v)) end)
%{
  "a" => #MapSet<["apple", "apricot"]>,
  "b" => #MapSet<["banana", "blackberry", "blueberry"]>,
  "k" => #MapSet<["kiwi"]>
}

With MapSet, short version

iex> list 
|> Enum.group_by(&String.first/1) 
|> Enum.reduce(%{}, &Map.put(&2, elem(&1, 0), MapSet.new(elem(&1, 1))))  
%{
  "a" => #MapSet<["apple", "apricot"]>,
  "b" => #MapSet<["banana", "blackberry", "blueberry"]>,
  "k" => #MapSet<["kiwi"]>
}