We do not currently support single sign in, but we are evaluating options for adding this capability so that users will be able to log in to your site, and then cross over to your forums without logging in or signing up. Right now, we offer Facebook login in order to streamline access for users. By logging in with Facebook, they gain immediate access without having to sign up. This makes it convenient for people who log in using Facebook. Hope that helps, and we'll keep you posted with updates regarding our support for single sign on.
Hi Minyu, I wanted to let you know that we have just released support for single sign-on (SSO) for premium accounts. We are providing SSO through an HTTP-encoded token containing user credentials. From within your application, you would construct a unique URL login link for the user. The link includes a base64-encoded JSON object containing the user fields and a message digest ensuring the validity of the data. If you would like to implement SSO for your community, please contact us to enable this for your account.