user_steps.rb

module UserHelpers
  def current_user_session
    return @current_user_session if defined?(@current_user_session)
    @current_user_session = UserSession.find
  end

  def current_user
    return @current_user if defined?(@current_user)
    @current_user = current_user_session && current_user_session.user
  end
end

World(UserHelpers)

def do_login(uname, pw)
  visit '/login'
  fill_in("username", :with => uname)
  fill_in("Password", :with => pw)
  click_button("Log in")
end

When /^I login with (.+)$/ do |unamepw|
  split = unamepw.split(/\//)
  do_login(split[0], split[1])
end