We already enrol everyone on joining automatically. Do we still need to enrol staff who opted out before our staging date?

If an employer is using contractual enrolment, workers will be enrolled under the contract of employment.  Once an individual enrolled via contractual enrolment ceases active membership however, the normal employer duties begin to apply – to monitor age and earnings, and auto enrol as appropriate.  These duties commence at staging date so if the person is not an active member at staging date, but meets the criteria for eligible jobholder, you would have to automatically enrol.  Exemptions from automatic enrolment are listed here.