If she is your spouse she would be admitted in L-2 status and is employment-authorized incident to that status, so the mill or any other employer may hire her. A partner who is not a spouse has no derivative status and needs a route of her own.
Marriage decides the derivative; status decides the start date
The L-2 category covers the spouse and unmarried children under 21 of an L-1 transferee. A spouse in L-2 status is treated as employment-authorized incident to status, which means she may work for the U.S. mill, for another employer or for herself without a separate application in most cases, although she may still choose to obtain an employment authorization document as convenient proof.
A common-law partner who is not legally married to the transferee cannot obtain L-2 status; her options would be a visitor admission with no work, or her own employment-based category if she has a listed profession and a U.S. employer. If the mill intends to hire the spouse for a bookkeeping or laboratory role, that is permitted once she is admitted in L-2 status, but she should not begin before then, and the mill's petition for the principal should not depend on her labour.
Children in L-2 status may attend school but may not work, and a child who reaches 21 loses derivative status.