When we have identified a device we need access to, we need to update our roster accordingly by annotating the contact with the corresponding groups. If a roster item already exists, we update it. If one does not exist, we add one. We also need to send a presence subscription to the corresponding device, if one is not already available, to be able to communicate with it:

   RosterItem Item = this.xmppClient[this.sensorJid]; 
   if (Item != null) 
   { 
         this.xmppClient.UpdateRosterItem(this.sensorJid, 
               Item.Name, this.AddReference(Item.Groups, 
               "Sensor", Thing.Node)); 
 
         if (Item.State != SubscriptionState.Both && 
               Item.State != SubscriptionState.To) 
         { 
               this.xmppClient.RequestPresenceSubscription( 
                     this.sensorJid); 
         } 
   } 
   else 
   { 
         this.xmppClient.AddRosterItem( 
               new RosterItem(this.sensorJid, string.Empty, 
               this.AddReference(null, "Sensor", Thing.Node))); 
 
         this.xmppClient.RequestPresenceSubscription( 
               this.sensorJid); 
   } 
} 
break;