Who uses it. The project owner or architect who has to certify an AIA pay application.
How they get in. Open the pay application, type their email into the reviewer field, and press Send to Owner/Architect for Approval. They open the link and certify without an account.
Treat this link as sensitive. Anyone holding it can certify the application — including anyone it gets forwarded to. Send it to one address, and re-send only when you want the reviewer changed.