> You probably cannot sign up for their 401K since you do > not work for them. You work for the company that issues > the W-2's. Unless you have just resigned from them and are > a full-fledged employee of the company with the 401K.

The company that issues the W-2's *is* the company that
offers the 401K. The question is whether or not (and if
so how) to participate.