Unless your employer picks up your defense, you need to respond separately. You are an independent, separately named party. However, you should definitely request that your employer defend and indemnify you. (I am assuming the accident occurred in the course and scope of your employment.) It won't cost your employer much more, if any, to have its lawyer defend you as well. There is a good chance your employer will agree to do so. If the facts warrant, your lawyer could perhaps ask the plaintiff to dismiss you and go after your employer only. However, I am not familar with the facts of your case.
This response is for information purpose only and does not constitute a legal advice. This response does not create an attorney-client relationship.
Presumably, your employer has a policy of liability insurance that will cover any damages that you have caused in the event you are determined to be liable for the accident. That insurance company will appoint a lawyer to represent both you and your employer and will defend you in the lawsuit. You should discuss with your employer as soon as possible the fact that you were served with these papers, and provide a copy of them to your employer. You should inquire of your employer to make sure your employer has notified its insurance company of the lawsuit. If you learn that your employer does not have insurance or for some reason is not going to notify the insurance company of the lawsuit (which would be a stupid move on its part), then you will need to consult with your own attorney to make sure that a timely answer is filed in response to the complaint. The summons will set forth how much time you have to file an answer (typically it is 45 days if you were sued in Wisconsin). Good luck.