CoGe's user account system is run using the iPlant authorization system. If you have an iPlant account, you can use those credentials to log into CoGe.
Pressing the "sign-in" link will take you to iPlant's authentication service. Type in your iPlantID and password. If you are validated, you will be returned to CoGe. When you return, your name and a logout link should appear instead of the login. Note: your name may not appear on CoGe upon returning from iPlant's authentication service. If that happens, just reload the CoGe webpage.
To register for an iPlant account: http://www.iplantcollaborative.org/forms/register-iplant-tools-services
See this page for step by step instructions: How to get a CoGe account