<script>
    $(document).ready(function () {
        $("#save").click(function () {
            var model = { Email: $(".email").text() };
            $.post("@Url.Content("~/Account/Register/")", JSON.stringify(model));
        })
    });
</script>

[HttpPost, AllowAnonymous, ValidateAntiForgeryToken]
public async Task<ActionResult> Register(RegisterViewModel model)
{
    if (ModelState.IsValid)
    {
        var user = new ApplicationUser { UserName = model.Email, Email = model.Email };
        var result = await UserManager.CreateAsync(user, model.Password);
        if (result.Succeeded)
        {
            await SignInManager.SignInAsync(user, isPersistent:false, rememberBrowser:false);
            return RedirectToAction("Index", "Home");
        }
        AddErrors(result);
    }

    return View(model);
}

var model = { Email: $(".email").text() };
$.post("@Url.Content("~/Account/Register/")", JSON.stringify(model));

$.post("@Url.Content("~/Account/Register/")");

$(document).ready(function () {
        $("#save").click(function () {
            var model = { Email: $("#em").val() };
            $.ajax({
                url: "@Url.Content("~/Account/RegisterAjax")",
                type: "POST",
                dataType: 'json',
                data: JSON.stringify(model),
                contentType: "application/json; charset=utf-8",
                cache: false,
                success: function (result) {
                    var validator = $("#registerForm").validate();
                    validator.showErrors(result.Errors);
                }
            });
        });
});

[HttpPost, AllowAnonymous]
public ActionResult RegisterAjax(RegisterViewModel model)
{
    var errors = new Dictionary<string, string>();

    if (true)
    {
        errors.Add("duplicate", "Такой email уже занят");
    }

    return new JsonResult { Data = new { Errors = errors } };
}