You can sign up for a new account at https://beta.stellaremperor.com/Account/SignUp. Sign up is currently free and requires an email address.
The client installer can be downloaded at http://beta.stellaremperor.com/StellarEmperorClient/StellarEmperor.exe. The installer will prompt installs of any necessary prerequisite software such as the .NET 4.6 Framework. There may be various updates throughout the beta phase, but updates should be delivered automatically.
The Stellar Emperor client currently requires a PC running Microsoft Windows Vista or newer with the Microsoft .NET 4.6 Framework installed.
Planetary Management may be executed through any HTML5 capable browser.