before(:each) for all tests except one

You can add metadata to tests that do not need to login, then evaluate that metadata in your before hook.

For example, two tests in the same file. One needs to login and one does not.

# foo_spec.rb
describe Foo do
  describe "#bar" do
    it "needs to log in" do
      expect(1).to eq 1
    end
  end
  describe "#baz" do
    it "needs to not log in", :logged_out do
      expect(1).to eq 1
    end
  end
end

So we added metadata to our it block. Next, we configure the before hook to evaluate the example's metadata.

config.before(:each) do |test|
  login(email, password) unless test.metadata[:logged_out]
  visit root_url
end

Now, each test will visit root_url but only the ones not tagged with :logged_out will call login.

RSpec calls these metadata based hooks filters. You can learn a bit more about them here.

Putting this kind of code into spec helper seems a bit odd. Feature specs is all you have? No unit tests? Even if it's the former, copy that code into individual specs that need it. If they're all in the same file, you can use contexts to prevent some duplication.

RSpec.describe 'something' do
  context 'specs with login' do
    before do 
      login(email, password)
      visit root_url
    end

    it { ... }
  end

  context 'specs without login' do
    it { ... }
  end
end

Global rspec config in spec_helper.rb is meant for other things. Ones that make sense for each and every spec. Like, for example, cleaning a database.

config.before :each do 
  DatabaseCleaner.clean
end