Add Users To Search Center Site. Here you can add new TABs remove them or re-order. In the Shared with dialog box click Invite people.
Under Advanced settings for external sharing select the checkbox next to Limit external sharing by domain. Go to the Policies tab. Now that you have your Search Center Site Collection you can move on to crawling content.
To add a new user.
Import from google search console. Open the Site menu by clicking the gear icon in the upper-right portion of the page and then click Site Permissions. The problem is that some people that have access to the child site is not allowed access to the rootsite and since the search center inherits user rights from the root site they are not allowed to search the child site. In a web browser go to the Search Center site.