The settings section is accessible only to Seller Admin. Go to "Settings" and choose the sub-section Authentication Management. One of the active Identity Providers can be a default. Choose the Identity Provider and select "Save". Users, having corresponding services, will be able to log-in to the platform.
Users can be invited with all enabled identity providers.
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 in the settings section.