Central Authentication Server
Redcell Synergy does not support RADIUS for authentication, however it does support LDAP (see the Redcell Synergy User Guide for how to enable LDAP), CAS, NTLM, OpenID, Open SSO and SIteminder. If you are not doing NTLM/LDAP/Active Directory, Central Authentication Server (CAS) is a widely used, open source central authentication solution.
This feature imports users with the default level of permissions. You must manually alter permissions and create groups if you want to differentiate between user permissions.
CAS can use various authentication schemes like LDAP, or Radius. Web applications like Redcell Synergy only need to know about the CAS server and have no knowledge about the various authentication protocols it uses to provide the final mappings.
One popular CAS Server is: http://www.jasig.org/cas
You can configure access to CAS in the Portal Settings CAS tab. Other tabs are available here for authentication too, for example LDAP and Active Directory (see the Redcell Synergy User Guide for how to enable LDAP).
Liferay provides foundation classes for Redcell Synergy’s web client. Liferay Wiki instructions about setting up CAS appear here: http://www.liferay.com/community/wiki/-/wiki/Main/CAS+Liferay+6+Integration.