There are a few ways to do this. Most are covered on this page
What I personally would probably do is define a SeleniumContext class and require this class in all my Step class constructors, then tell SpecFlow's IOC to use the same instance in every scenario:
First create the class to hold the selenium driver instance
public class SeleniumContext
{
     public SeleniumContext()
     {
          //create the selenium context
          WebDriver = new ...create the flavour of web driver you want
     }
     public IWebDriver WebDriver{get; private set;} 
}
then setup the IOC to return the same instance every time
[Binding]
public class BeforeAllTests
{
    private readonly IObjectContainer objectContainer;
    private static SeleniumContext seleniumContext ;
    public BeforeAllTests(IObjectContainer container)
    {
        this.objectContainer = container;
    }
    [BeforeTestRun]
    public static void RunBeforeAllTests()
    {
        seleniumContext = new SeleniumContext();
     }
    [BeforeScenario]
    public void RunBeforeScenario()
    {            
        objectContainer.RegisterInstanceAs<SeleniumContext>(seleniumContext );
    }
}
Then ensure your step classes always ask for the context in their constructors (you need to do this in every step class you have)
[Bindings]
public class MySteps
{
    private SeleniumContext seleniumContext;
    public MyClass(SeleniumContext seleniumContext)
    {
         //save the context so you can use it in your tests
         this.seleniumContext = seleniumContext;
    }
    //then just use the seleniumContext.WebDriver in your tests
}
alternatively if you are already storing the instance in the feature context then you can just use the BeforeFeature hook to save the same instance:
[Binding]
public class BeforeAllTests
{
    private static WebDriver webDriver;
    [BeforeTestRun]
    public static void RunBeforeAllTests()
    {
        webDriver = new WebDriver();
     }
    [BeforeFeature]
    public static void RunBeforeFeature()
    {
        FeatureContext["WebDriver"] = webDriver;
     }
}