Rainforest sends many testers at once to your site so your runs are fast. Use Step Variables to avoid race conditions and shared state between these testers.
To authenticate testers so that they can test features behind your app's login, most Rainforest customers either get each tester to sign up for a fresh account or login to a test account. Once this stops working, you can use step variables to ensure each tester is logged into a unique account.
First, generate sufficient logins for your test suite then add them to your account. Finally, update your login test with your shiny new step variables.