One supplier user to access multiple suppliers' documents

Summary

One supplier user to access multiple suppliers' documents

Content

We procure material from a big conglomerate. For different tax payer ID's, we defined different suppliers. But they all belong to this parent supplier. 

Now the question is how can we create one supplier user who would have access to the RFQ's and PO's for all the suppliers for this conglomerate?

For example, if there are 5 suppliers for this conglomerate and they are 3 users, how can these 3 users access the RFQ's and PO's for all these 5 suppliers? Note we cannot create 15 different supplier user accounts as we use email ID's as the SSO.