To ensure that content on Microsoft SharePoint Online sites can be shared only with new and existing guest users, you need to limit the external sharing settings in the SharePoint admin center. Here are the steps to follow:
* Sign in to the SharePoint admin center with your admin credentials.
* Go to Policies > Sharing.
* Under External sharing, drag the SharePoint slider down to New and existing guests. (The OneDrive slider is dependent on SharePoint and will also be set to New and existing guests.)
* Under More external sharing settings, check the box Allow only users in specific security groups to share externally and select Manage security groups.
* Add the security groups that contain the users who are allowed to share content with guest users. You can also create a new security group if needed.
* Click Save to apply the settings.