As a federal contractor, must I notify my existing employees individually that I must verify them in E-Verify?

No. The E-Verify MOU only requires employers to “display the notices supplied by DHS in a prominent place that is clearly visible to prospective employees and all employees who are to be verified through the system.” To comply with this requirement, employers should display the E‑Verify Participation and Right to Work posters in a prominent location where both prospective and existing employees can see it. These notices can also be found in the Essential Resources section of E-Verify.