Settings section is accessible only in "Seller view". Log-in to your platform. Go to "Settings" and choose the sub-section Authentication Management. Here you can see the list of supported identity service providers. You can choose from one option to several. After selecting the Identity Provider(s), press "Save". Users, having corresponding services, will be able to log-in to the platform.
Afterward, go to the "Users" tab → Invite Users. Fill in the necessary fields. In more details read the article User Invitation for non-Microsoft partners
In the Identity Provider section, you will have the identity service provider, which you have selected above.