Symptoms

When logging into to K2 SmartForms, users are prompted for credentials everytime they access aform. Site Url has been added to trusted site zone and IE is set to "Automatic Login only in Intranet Zone". This happens for some users but not all
 

Diagnoses

As the K2 site was added to the Trusted Sites, the Internet Options > Authentication should be configured for "Automatic logon with current user name and password" as per documentation:

http://help.k2.com/onlinehelp/k2blackpearl/icg/current/webframe.html_troubleshooting_settingsforie.html
 

Resolution

After making the configuration changes, the issue has been resolved.