In addition to a regular signup process and login with your username and password, it is now also possible to signup / login directly with your Github account (using OAuth authentication).
Selecting Github brings you to:
Where you need to sign in with your Github account. If you were already signed in, then you can just accept and your account will be created.
Then on subsequent logins select the Github issue as well:
Associating existing account to Github
In your profile settings you can associate your existing account with Github and then Github will be used for subsequent logins: