If you are looking to use procurement and sourcing good chance you are working with suppliers. It might help efficiency to allow suppliers to be able to collaborate in the same system. With D365 you can allow a vendor to access the system and give basic access order and invoice flows.
The setup requires a contact for a vendor to be enabled and setup with a user. This is where the internal collaboration with your IT organization will come in because it will requires an Azure AD user to be available for the external user.
Here is a quick overview.
D365 1611 Update5 (7.0.4475.35138)
You can read further here.
If you are looking for AX2012 you can refer to this post.