Handling user logins while using a multiple schema design to support multi-tenancy
Your options are:


Use roles, and SET search_path on each role so it looks in the appropriate organization's schema. While in many ways the cleanest approach from a database perspective, this is a pain when giving orgs the right to manage their own users.
Require users to specify their organisation at login time, probably with user@domain style logins, or using mycompany.myhostedapp.com style URLs. Then look up the org by @domain suffix before authing the user against that org. That's probably what I'd do.
Create a view that's a union of all company user tables and look users up in this. Likely to be a performance nightmare, and how will you handle duplicates?
Create a parent table for all user tables, make all the user tables inherit it, and query the parent table. Just a slightly more efficient version of the view approach, not recommended.


I strongly suggest an authentication design that carries the company identity of the user along with their login request.