Legally the first notice to employees can be issued after your staging/duties start date, however, it would be wise to notify your employees in advance about your chosen provider and more information on what is to come. This will help reduce the burden of dealing with questions once the processes has started.
How much time is needed to set up a scheme to meet a staging date?
How far ahead should employers start preparing for their staging date?
If an employer is using postponement, do they still need to be ready at staging date?
Auto enrolment staging dates