Due to a known Privilege Escalation exploit the addUserToGroup method available through the Attlassian Group Manager has been restricted. Attempts to use the method will result in a similar Script Exception:
Script Execution failure: ScriptException: An unhandled exception occurred during script execution, it could a script fault, or an attempt to use a Class not specifically cleared, see cause:
java.security.AccessControlException: addUserToGroup method restricted to prevent Privilege Escalation:
Workaround
JEMH has an Auto Join groups feature, which allows users to be joined to groups automatically during issue processing for more info please see: