Follow the following steps:

1. Create a Gemini admin user with the username of YOUR DOMAIN NAME\USER NAME
2. Change the web.config file from Forms authentication to Windows: <authentication mode="Forms"> change to <authentication mode="Windows">
3. In IIS change the directory security to use integrated windows authentication only.
4. For Windows Authentication, in IIS 'Allow Anonymous User' option should be unchecked/disabled.

Note: For Gemini Scheduler and Gemini add-on products to work with Windows Authentication, please implement the changes recommended in this Microsoft KB Article and then disable anonymous access in IIS.
That is it, after this add your users in the same way (DOMAIN\User) and they will be logged on automatically.