This new feature allows admins to easily add M365 users to their roster without having to approve them manually.

There are two ways to enable this feature, at the MSP level and at the single company level:

Enabling Auto-Approve at the Child Company level

If you want to enable this feature for a single company you could navigate to the child company settings and look at the section M365 Users Sync. From that section you should click on the Auto-Approve checkbox that is being shown at the bottom part of that section. Once this checkbox has been clicked a confirmation modal will appear to ensure you really want to enable this option.

Enabling Auto-Approve at MSP level

There's an option on the MSP portal that allow MSP owners to enable the Auto-Add feature for all their child companies. 

This action is located at the MSP settings page and is enabled by clicking on the Switch icon that is being show inside the section called "M365 Auto Approve".  After clicking on that switch the app will display a confirmation modal to ensure you really want to do this action.

By enabling the auto-approve feature at the MSP level all your child companies will be automatically updated and will ALL auto-approve new users, so the new users won't need a manual approval. 

To keep in mind: If at the moment of enabling the Auto Add feature there are one or more users on the pending for approval list, they will remain there and will need a manual approval from the admin, so it means that the auto-approve will apply only for new users.

If you try to create a new company and the Auto-Approve feature is already enabled at the MSP level then that new company will have the auto-approve checkbox enabled as default, however you can uncheck it at any moment.

You need to be careful when enabling the Auto-Approve feature, remember that every user added on the M365 tenant will be automatically included into your roster, so it will impact your company billing.