Notes/Domino 4 and 5 Forum

i'am an administrator on domino server 5.x, in my comagny i have to make restriction on user, in other words users musn't send message to any bady, evry user has a list of contact, the other contacts are forbiden for him, how can i do it, make restriction on user, please help me with screen image if you can, i am a new in the administration of domino