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

    if (!ModelState.IsValid)
    {
        var messages = ModelState.Values.SelectMany(v => v.Errors);
    }
        
    return new JsonResult { Data = new { Errors = errors } };
}

@model WebApplication15.Models.RegisterViewModel
@{
    ViewBag.Title = "Register";
}

<h2>@ViewBag.Title.</h2>

@using (Html.BeginForm("Register", "Account", FormMethod.Post, new { @class = "form-horizontal", role = "form" }))
{
    @Html.AntiForgeryToken()
    <h4>Create a new account.</h4>
    <hr />
    @Html.ValidationSummary("", new { @class = "text-danger" })
    <div class="form-group">
        @Html.LabelFor(m => m.Email, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.TextBoxFor(m => m.Email, new { id = "email", @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.Password, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.PasswordFor(m => m.Password, new { id = "p", @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.ConfirmPassword, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.PasswordFor(m => m.ConfirmPassword, new { id = "p2", @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input id="save" type="submit" class="btn btn-default" value="Register" />
        </div>
    </div>
}

<script>
    $(document).ready(function () {
        $("#save").click(function () {
            var model = { Email: $("#email").val(), Password: $("#p").val(), ConfirmPassword: $("#p2").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);
                }
            });
        });
    });
</script>

public class RegisterViewModel : IValidatableObject
{
    [Required, EmailAddress, Display(Name = "Email")]
    public string Email { get; set; }

    [Required, DataType(DataType.Password), Display(Name = "Password")]
    [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
    public string Password { get; set; }

    [DataType(DataType.Password), Display(Name = "Confirm password")]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        if (EmailExists(Email))
        {
            yield return new ValidationResult("Такой email уже существует");
        }
    }

    public bool EmailExists(string email)
    {
        return true;
    }
}