Delegation Membership of a group

Hi!

I created a control delegation for some users and allowed Modify the membership of a group but users cannot do it. The message is : You do not have the permission to modify the group...

I checked OU permissions for my group and I have Read/Write Members for Descendant Group Objects.

I configured the delegation with tasks like create, delete users, reset pwd, create delete manage groups...

Do I have any conflit with permissions? 

Could you please help me?

Thanks