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When you start a site with a Premium subscription trial to a Cloud product, you automatically create a “site-admins” group that contains your account as the only member. The “site-admins” group has the site admin role, which allows you to manage the site settings, products, users, and groups. You can also add other users to the “site-admins” group if you want to delegate some of the site administration tasks.

References = Create and update groups and Manage site-level permissions