Scope it from the phone log
The most reliable way to specify a portal is to spend a fortnight counting what customers currently ask for and in what volume, then build that list in order.
It produces a less exciting specification than a workshop does. It also produces a portal that removes work, which is the only reason to build one. Features that look good in a demo and answer a question nobody asks are the main way these projects waste money.
Identity is the hard part
The interface is rarely what takes the time. Company hierarchies, user permissions, who can see contracted pricing, who can commit spend, and how all of that maps to records in your ERP — that is where portal projects overrun.
It is worth deciding early whether customers manage their own users. Letting them is more work up front and dramatically less work forever afterwards.
Where this sits
For businesses selling to other businesses this is usually delivered alongside B2B commerce rather than separately. It depends on integration work for orders, invoices and shipments, and sits on a storefront built under eCommerce development.