I'm not sure what exactly you're being sued for, but some other posts seem to indicate that it has something to do with giving adequate notice.
I don't think this is the case, since your failure to give adequate notice probably would not result in a lawsuit against your prospective employer.
It might be that they are suing you and prospective employer for tortious interference with a contract. Some states, including Wisconsin, may read an employment contract into at-will employment if certain conditions apply. Therefore, when you took another job with your prospective employer, you may have breached your "contract" with your former employer, and they may have grounds to sue whomever induced the breach.
You need to find out what exactly the claim is, and hire an attorney. Your former employer has probably already consulted with its in-house counsel, so it might not be a good idea to do this on your own.
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I'm not sure what exactly you're being sued for, but some other posts seem to indicate that it has something to do with giving adequate notice. I don't think this is the case, since your failure to give adequate notice probably would not result in a lawsuit against your prospective employer. It might be that they are suing you and prospective employer for tortious interference with a contract. Some states, including Wisconsin, may read an employment contract into at-will employment if certain conditions apply. Therefore, when you took another job with your prospective employer, you may have breached your "contract" with your former employer, and they may have grounds to sue whomever induced the breach. You need to find out what exactly the claim is, and hire an attorney. Your former employer has probably already consulted with its in-house counsel, so it might not be a good idea to do this on your own.