Site collection administrators can be added via the SharePoint admin center user interface. To do so, use the following procedure:
- Navigate to the SharePoint Admin Center (https://admin.microsoft.com), expand Admin centers, and select SharePoint.
- In the left navigation pane of the SharePoint admin center, select Active sites.
- Select a site, and then select the Owners drop-down menu from the menu bar. For classic sites, you'll be given the following options: Change primary admin and Change admins. For modern sites, you'll see the Change group owners option, as shown in the following screenshot:
- Modify the admin membership. If you are modifying a classic site and select a new primary admin, the current primary admin will continue to be listed as an additional admin. When finished modifying the membership, click Save at the bottom of the flyout:
