This is what my source says but hold for cbg to come back & see what the person she talks to has to say.
At one time, the only way you could join your company's 401(k) plan, 403(b) plan, or 457(b) plan was to put pen to paper and sign yourself up by filling out the appropriate forms. Now, though, in an effort to help participants increase their retirement savings, some employers have begun enrolling their employees automatically. With automatic enrollment, you don't fill out a form to opt into your company's retirement plan; you only fill out a form to opt out of it. You need to ask your employer for an opt out form if you do not wish to participate in their retirement plan. They cannot require you to participate though very few employees will opt out.

