if you have an existing masking view then a storage group must be there already...Do you want to know how to add the additional device to a storage group/masking view?
You can only add devices to an existing masking view, not storage groups with 5874. SE 7.4 and 5876 allows you to have cascaded storage groups so you could then add an SG into an SG in a masking view. So in your case you would to just need to create a new masking view for the new SG.
@ Saurabh No .. i wanted to add a newly created storage group to the existing masking view and i already had a SG added to the masking view.