This Microsoft 365 post will show you how to prevent and disable non-admin users from adding guest users to a Microsoft 365 tenant.
By default, any Microsoft 365 user has the capacity and rights to add guests to the organisation and access resources.
This setting might be OK for small organizations without IT knowledge, helping them get going and collaborate with external people. However, this configuration can create a major security incident in large organizations.
To disable external sharing and prevent Microsoft 365 users from adding guest users, open the Microsoft 365 admin center.
Click on Settings
Click on Org Settings
Click on Sharing (Under the Security & Privacy tab)
In the Sharing configuration page, un-tick Let users add mew guests to the organization.