This article provides information regarding SSO (Single Sign-On) login with SAML (Security Assertion Markup Language), which requires users to be federated if Merge Local User is enabled.
Jive Interactive Intranet - Hosted/AWS/Cloud Version
Enable the Username Identity setting if you have existing users in Jive and you are newly implementing SAML. (You do not need to enable it if all your accounts are created through SSO auto-provisioning.)
Jive uses a permanent, unique identifier (External ID) to connect existing users with their SSO login. If users have never logged in using SSO, they will not have an associated External ID. When Username Identity is enabled, Jive will map any existing federated users to an existing user account using their username or email address during their first SSO login.
To automatically federate existing users on login, you should also enable Merge Local Users. If you use Username Identity without also enabling Merge Local Users, make sure your existing users are marked as federated users. Otherwise, un-federated users will not be synchronized.