And to answer your question: https://www.enterpriseready.io
Basically, b2b app = classic app + deep users management with Gmail SSO and/or CSV import and/or windows UAC integration (depends a lot of your target) + deep roles/permissions management + integrations with existing apps/saas + reports and alerting + automatic billing with Stripe or whatever.
Note that you can do most of those manually at the beginning. Send the reports and the bills by email, add the new companies/users manually…