Add join/leave group

lib/mobilizon/actors/actors.ex

@@ -162,6 +162,12 @@ defmodule Mobilizon.Actors do
     )
   end
 
+  @doc """
+  Get the default member role depending on the actor openness
+  """
+  def get_default_member_role(%Actor{openness: :open}), do: :member
+  def get_default_member_role(%Actor{}), do: :not_approved
+
   @doc """
   Get a group by it's title
   """