Purchasing groups - cross company purchasing

Dear SRM experts,

Currently we are using R/3 MM as procurement system and purchasing organizations are assigned at company code level.Now we are implementing SRM extended classic scenario and business has got a requirement, where SRM should allow cross company level purchasing for some purchasing groups.

Could some one help me what is the Org. structure hierarchy that I need to create in order to fulfil this requirement?

Thanks in advance,

Krish