mvc3 checkbox checked rows in the web grid

1. Ensure that your webgrid is wrapped inside a form.

@using (Html.BeginForm("Assign","Home"))
  {
  }

In the webgrid columns add a column with check box as follows which I think already you have.

grid.Column(header: "Assign?", format: @<text><input class="check-box"  id="assignChkBx"name="assignChkBx" type="checkbox" value="@item.Id"/></text>)

2. At Controller Retrieve objects that are checked using id values as follows::

[HttpPost]
        public ActionResult Assign(FormCollection form)
        {
            var ch = form.GetValues("assignChkBx");
            foreach (var id in ch)
            {
              //...Get object using id
              //...Call your method
            }
            return View();
        }