Besides the possibility of integrating them using a SAML IdP, you can also integrate them through a custom social connection. This would imply that your partners would have to have an OAuth2 implementation. Although there’s still a requirement, it’s arguable easier to have an OAuth2 implementation than a SAML one.
For more information about the above option check:
As an additional alternative to all of the above it’s also possible to configure custom database connections. In this situation, although the connection type refers to a database, it’s not strictly necessary that an actual typical database exists.
For example, if your partners provide an HTTP API that can validate a username and password credentials associated to their users you would be able to implement a custom database connection that calls into this API in order to authenticate the user. Have in mind that with custom database connection you would most likely have to have additional work on your side because now you would need to know ahead of time which connection does the user authenticate to.