ESS/MSS Portal Group vs ESS Mss Portal

Hi Everyone,

I am completely newbie to SAP ESS/MSS, I was asked question what is difference between ESS/MSS portal Group and ESS/MSS Portal. Would anyone be able to clarify this to me, also it would be helpful if you could provide me information on how to setup a new ESS/MSS portal on existing platform.

regards

Zafi