I've decided to add user authentication to my application using
restful_authentication plugin. It has generated Account model with
such fragment of code:

def create
    @account =[:account])
    @account.register! if @account && @account.valid?
    success = @account && @account.valid?
    if success && @account.errors.empty?
      flash[:notice] = "Thanks for signing up! We're sending you an
email with your activation code."
      flash[:error] = "We couldn't set up that account, sorry.
Please try again, or contact an admin (link is above)."
      render :action => 'new'

Everything is clear to me except @account.register! method. I cannot
find any documentation about it. Where should I seek?

From the README
-- clip --
* --stateful: Builds in support for acts_as_state_machine and
  activation code. (@--stateful@ implies @--include-activation@).
Based on the
  idea at [[
stateful_authentication]]. Passing
  @--skip-migration@ will skip the user migration, and @--skip-routes@
will skip
  resource generation -- both useful if you've already run this
-- clip --

Using --stateful when running the restful_authentication generator it
builds in support for acts_as_state_machine. If you are not using that
plugin then generate restful_authentication without the --stateful

@user.register! is a state transition. If you are unfamiliar with
acts_as_state_machine, either read up on that and use it, or don't use
the --stateful option.

P.S. @account in your case instead of @user.

Thanks. I'll read about that.