Fix mentioning of private groups.

app/models/concerns/mentionable.rb

@@ -42,10 +42,10 @@ module Mentionable
     SystemNoteService.cross_reference_exists?(target, local_reference)
   end
 
-  def mentioned_users(current_user = nil, p = project)
+  def mentioned_users(current_user = nil)
     return [] if mentionable_text.blank?
 
-    ext = Gitlab::ReferenceExtractor.new(p, current_user)
+    ext = Gitlab::ReferenceExtractor.new(self.project, current_user)
     ext.analyze(mentionable_text)
     ext.users.uniq
   end

app/models/concerns/participable.rb

@@ -40,7 +40,7 @@ module Participable
       meth = method(attr)
 
       value = 
-        if meth.arity == 1
+        if meth.arity == 1 || meth.arity == -1
           meth.call(current_user)
         else
           meth.call