def participants(exclude_email=nil)
      ps = []
      self.posts.each do |p|
        if p.author_name && p.author_email
          ps << [p.author_name, p.author_email]
        end
      end
      ps.delete_if{|p| p[1] == exclude_email}.uniq
    end
def participants(exclude_email=nil)
      ps = []
      self.posts.each do |p|
        if p.author_name && p.author_email
          ps << [p.author_name, p.author_email]
        end
      end
      ps.delete_if{|p| p[1] == exclude_email}.uniq
    end
returns an array of participants for the discussion