public ActionResult Admin(uint id, FormCollection collection) {
    var username = collection.Get("username");
    var password = collection.Get("password");
    Helper.CreateUser(username,password);
    return View("AdministerUsers");
}

<% using (Html.BeginForm()){ %>
    <fieldset>
    <legend>Fields</legend>
    <label for="username">username</label>
    <%= Html.TextBox("username") %>
    <label for="password">password:</label>
    <%= Html.TextBox("password") %>
    </fieldset>
    <input type="submit" value="Add User" name="submitUser" />
<% } %>

routes.MapRoute(
    "Admin",
    "Admin/{id}",
    new { controller = "Administration", action = "Admin"}
);

public class ViewModel
    {
      public string Username {get; set;}
      public string Password {get; set;}
    }

<%@ Page Title="" Language="C#" Inherits="System.Web.Mvc.ViewPage<ViewModel>" %>  //the ViewModel will need to have it's fully qualified name here

<% using (Html.BeginForm()){ %>

    <%= Html.LabelFor(m => m.Username) %>
    <%= Html.TextBoxFor(m => m.Username) %>

    <%= Html.Label(m => m.Password) %>
    <%= Html.TextBoxFor(m => m.Password) %>

    <input type="submit" value="Add User" name="submitUser" />
<% } %>

[HttpPost]
public ActionResult Admin(ViewModel model)
{
    var username = model.Username;
    var password = model.Password;
    Helper.CreateUser(username,password);
    return View("AdministerUsers");
}

public ActionResult Admin(uint id, string username, string password)
 {
      // .. Do your stuff
 }