It has been asked when MeteorJS will have auth in place... I think everyone is anxious to hear back on this for such a fantastic platform.
According to the branch author they just finished up the first chunk today and are ready for input / testing:
If you want to get started with what's already there you can use Meteorite and create a project using the auth branch of Meteor with:
mrt create newappname --branch auth
Then you can see how to add various authentication schemes and render pre-generated templates for allowing users to log in by reading through the Meteor Wiki here: https://github.com/meteor/meteor/wiki/Getting-started-with-Auth
For example, you get a template that provides the whole login button with form by placing something like {{> loginButtons}} into one of your templates.
Remember that the implementation may still change and you'll want to switch your app back to master once auth gets merged in at a later stage to keep things more up-to-date. There are apps out there using auth and they work fine. For an example, see Atmosphere.