Do employers need to obtain employees’ consent for them to join a qualifying automatic enrolment pension scheme?

No.  A key feature of the automatic enrolment provisions is that eligible jobholders must be automatically enrolled into a qualifying scheme and that no conditions of entry can be imposed. Automatic enrolment is compulsory, although jobholders may choose to opt out of the scheme once they have been enrolled.  Eligible jobholders must not be required to provide information to join a scheme or sign an application form.

Exemptions from automatic enrolment are listed here