This is a snippet that will create a new user in jenkins and if security has been disabled , it will enable it :)
import jenkins.model.*
import hudson.security.*
def instance = Jenkins.getInstance()
def hudsonRealm = new HudsonPrivateSecurityRealm(false)
hudsonRealm.createAccount("MyUSERNAME","MyPASSWORD")
instance.setSecurityRealm(hudsonRealm)