If you need to auhtenticate accross different domains:
You can implement your own security token service (like facebook, google does) Here is some ready to use implementation: http://thinktecture.github.io/Thinktecture.IdentityServer.v2/
If the sites are running on the same domain (subdomain), then you can try to share an authentication cookie within these domains.
An explaining article: http://www.codeproject.com/Articles/106439/Single-Sign-On-SSO-for-cross-domain-ASP-NET-applic