Admins on the ATS will generally become admins on Onboarding. To ensure an ATS admin is an Onboarding admin too (or to change this access), edit their employee record on Onboarding and view their 'Role'. You can do this by navigating to the Employees page on Onboarding and clicking that employee's name. Next, click their Management page and view and/or change their 'Role'.
Full and limited access users on the ATS will generally be set as Managers on Onboarding. They will have access to employees in their location. Admins can view which location a manager is in on that manager's management page. If that manager needs an additional location or department access, it can be added by an Onboarding admin from the Account > Settings page.
An exception to these rules: Any CareerPlug ATS user who was hired from a CareerPlug job (no matter their ATS access level — admin, manager, or no recruiting access) will become an 'Employee' on Onboarding. It's a good idea to check someone's access level on Onboarding if they do not seem to have the correct permissions. To do this, navigate to the Employees page on Onboarding and click that employee's name. Next, click their Management page and view and/or change their 'Role'.
Those with no recruiting access on the ATS will be Employees, who only have access to their information and tasks.
More info: What do the levels of access mean in the Onboarding system?
If you have an employee who is not an ATS user and you want to add them manually to Onboarding, you can do so by clicking the 'Add Employee' link on the Employees page of Onboarding.
When you make a hire or add a user to the Users page on the ATS after you have activated Onboarding, that user will be transferred over to Onboarding automatically as long as they log in and access Onboarding.