Ok, you have a couple of questions, so I'll work through them:
Then, using the [BeforeScenario("myTag")] should do the trick.
The BeforeScenario hook attribute is used to run some code before the scenario executes. It's often used to set-up the environment for the scenario (e.g. populate the test database with pertinent data); if used for this purpose, then the use of AfterScenario can also be used to clean-up the result of BeforeScenario.
The hooks allows for the execution of a subset of the tests to be executed following certain rules. So, a When method could then be executed with a predefined context
If I understand you correctly, you want to be able to use a tag to control when a step within the scenario can be run/not-run. This is not possible with SpecFlow's hook attributes; there is a BeforeStep hook but this only enables you to execute code before the step is run, it doesn't allow the step to be ignored.
Are all the "too short password", "no credentials provided" considered different usage scenarios, or are they something else which could only fit somewhere else in the code, like the unit tests themselves?
In your example, yes these are different scenarios for your "Register a new user" feature. If you are taking a strict BDD approach to your development, then with your "outside-in inside-out" development approach you will also implement unit tests (by falling back to TDD as part of the BDD process) which will also cover the "too short password" and "no credentials provided" validation.
As for your scenario:
When I have entered my desired username "UserName" and password "password"
Instead of using this, use:
When I enter my username "UserName"
And I enter my password "password"
By doing this you will be able to re-use "When I enter my password" in "Register a user using a password that is too short". This leads me onto:
And I have provided a password that is too short "allo"
There is no need to have a separate step which states the password is too short. Just re-use:
When I enter my password "allo"
For the same reason, don't use:
When I click on the Register button with no credentials entered
just reuse:
When I click on the Register button