To configure a secondary site collection administrator for an individual OneDrive for Business site, follow these steps:
- Navigate to the SharePoint admin center Classic Features administration center (https://admin.microsoft.com | Admin centers | SharePoint | Classic Features).
- Launch the User Profile Service application from the classic SharePoint admin page by selecting User profiles.
- Select Manage User Profiles.
- In the Find profiles box, enter a username and click Find to search for them:
- Hover over the account name, select the down arrow that appears, and select Manage site collection owners:
- In the Site Collection Administrators box, enter a name and click Check names. Then, select OK:
Now, the added user should be able to access the site.