ASP.NET membership model for multiple company accounts, each of which has multiple users

Question

Tyring to create a membership system for multiple company accounts, each of which has multiple users (as opposed to only unique users who are the default ASP.NET membership providers).

This seems to be a very common scenario, but cannot find any recommendations. How is this usually implemented? ARGH !!

Company Account A [CompanyID]; User 1 [UserID]; User 2 [UserID]; ...

Company Account B [CompanyID]; User 3 [UserID]; User 4 [UserID]; ...

Can I use ASP.NET 2.0 Memberhip Provider (as is or change?), Or do I need to roll from scratch?

Planning a SaaS application in ASP.NET 4.x Web Forms, C #, SQL Server 2008/12, a common general DB + schema. Clients will register in accounts (with automatic reservation), and then configure their users on their own (or approve registration).

I believe that I understand the DB aspect (CompanyID + UserID), not the membership provider. I plan to use the email address as a UserName (always unique), which will then determine who the company belongs to, so there is no need to use URLs.

Any guidance is much appreciated!