import com.atlassian.jira.component.ComponentAccessor
def groupManager = ComponentAccessor.getGroupManager()
!(groupManager.isUserInGroup(issue.assignee?.name, 'groupname'))
/* the same for advanced script below */
import com.atlassian.jira.component.ComponentAccessor
passesCondition = true
def groupManager = ComponentAccessor.getGroupManager()
if (groupManager.isUserInGroup(issue.assignee?.name, 'jira-administrators')){
passesCondition = false
}