It depends on what you want the user be able to do. If you want them only to search for items and to use public items, enabling the "anonymuous access" on your Portal will do.
However, if you want to put them into groups / assign roles to them, you need an identity store. This can be the ArcGIS Portal identity store but it can also be your organization's enterprise identity store. When using an identity provider with SAML 2.0 Single Sign On support, you can give the user a single sign on experience (what is what your are looking for).
To read more about the access options and identity store integration: Managing access to your portal—Portal for ArcGIS (10.7 and 10.7.1) | ArcGIS Enterprise
More about SAML 2.0 Single Sign On with Portal: Configure a SAML-compliant identity provider with your portal—Portal for ArcGIS (10.7 and 10.7.1) | ...
