Maybe they're filing a separate suit against your partner, maybe they've settled out with him, or there may be any one of a number of strategic or other reasons for them not to have named him on this summons. Answer it ASAP and read my below link.
I do wish you the best.
This answer is provided for informational purposes only. Actual legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.
You should strongly consider hiring a lawyer to represent you in this matter at the very least you should consult with a lawyer to discuss your options.
If you chose to go it alone, keep in mind you have 30 days (45 days if you pay court costs) from the date you are served with the lawsuit to answer. Certain affirmative defenses are waived if you do not raise them in your answer. If you feel that other defendants should be included in the suit there are mechanisms for defendants to add more defendants. Probably not going to be easy for a non-lawyer. I would call a lawyer, however, many courthouses have legal libraries that have materials that can help you navigate the difficulties of defending yourself in a suit.