MVC3 unobtrusive validation group of inputs

You could write a custom attribute:

public class AtLeastOneRequiredAttribute : ValidationAttribute, IClientValidatable
{
    private readonly string[] _properties;
    public AtLeastOneRequiredAttribute(params string[] properties)
    {
        _properties = properties;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (_properties == null || _properties.Length < 1)
        {
            return null;
        }

        foreach (var property in _properties)
        {
            var propertyInfo = validationContext.ObjectType.GetProperty(property);
            if (propertyInfo == null)
            {
                return new ValidationResult(string.Format("unknown property {0}", property));
            }

            var propertyValue = propertyInfo.GetValue(validationContext.ObjectInstance, null);
            if (propertyValue is string && !string.IsNullOrEmpty(propertyValue as string))
            {
                return null;
            }

            if (propertyValue != null)
            {
                return null;
            }
        }

        return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        var rule = new ModelClientValidationRule
        {
            ErrorMessage = ErrorMessage,
            ValidationType = "atleastonerequired"
        };
        rule.ValidationParameters["properties"] = string.Join(",", _properties);

        yield return rule;
    }
}

which could be used to decorate one of your view model properties (the one you want to get highlighted if validation fails):

public class MyViewModel
{
    [AtLeastOneRequired("Email", "Fax", "Phone", ErrorMessage = "At least Email, Fax or Phone is required")]
    public string Email { get; set; }
    public string Fax { get; set; }
    public string Phone { get; set; }
}

and then a simple controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new MyViewModel();
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(MyViewModel model)
    {
        return View(model);
    }
}

Rendering the following view which will take care of defining the custom client side validator adapter:

@model MyViewModel

<script src="https://stackoverflow.com/questions/5816313/@Url.Content("~/Scripts/jquery.validate.js")" type="text/javascript"></script>
<script src="https://stackoverflow.com/questions/5816313/@Url.Content("~/Scripts/jquery.validate.unobtrusive.js")" type="text/javascript"></script>
<script type="text/javascript">
    jQuery.validator.unobtrusive.adapters.add(
        'atleastonerequired', ['properties'], function (options) {
            options.rules['atleastonerequired'] = options.params;
            options.messages['atleastonerequired'] = options.message;
        }
    );

    jQuery.validator.addMethod('atleastonerequired', function (value, element, params) {
        var properties = params.properties.split(',');
        var values = $.map(properties, function (property, index) {
            var val = $('#' + property).val();
            return val != '' ? val : null;
        });
        return values.length > 0;
    }, '');
</script>

@using (Html.BeginForm())
{
    @Html.ValidationSummary(false)

    <div>
        @Html.LabelFor(x => x.Email)
        @Html.EditorFor(x => x.Email)
    </div>

    <div>
        @Html.LabelFor(x => x.Fax)
        @Html.EditorFor(x => x.Fax)
    </div>

    <div>
        @Html.LabelFor(x => x.Phone)
        @Html.EditorFor(x => x.Phone)
    </div>

    <input type="submit" value="OK" />
}

Of course the custom adapter and validator rule should be externalized into a separate javascript file to avoid mixing script with markup.