Login for multisites

I want to design a multisite that has a home page with a sidebar login box that when the registered user logins it will take them to a predetermined site. I will register the users and issue their login details so they dont need access to a profile page. Is there a plugin that will manage this, i.e. user A gets directed to site A, user B gets directed to site B. I’m not able to search for the plugin as i’m not sure what this process is called. Hope my description is clear.