Need to check an extra table on login

I am writing a plugin at the moment that grants 24 hours access to wordpress. It stores temporary users in a separate table to the main users table. I need to add a function that forces the login to look at the extra table to check is a user is valid and is still granted access. Any ideas?
As well as this, once I add a user to the database via a form in the sidebar they are submitted fine but I cannot return to the page that came from. What am I meant to do to return? Thanks in advance