You can define the additional reviewers in the "Additional Reviewers" tab of the Part:

Once the Part is collected by a Design Verification workflow, this will automatically assign them a control activity that must be validated to complete the workflow successfully.

Be careful!! In case you add a group as additional reviewer, an additional control activity will be assigned to each member of this group and the workflow will only be completed once every single user will have validated the control activity they have been assigned to.

Complete documentation: Design Verification Workflow