ValidationGroupBehavior.cs

...
namespace XamarinFormValidation.Behaviors
{
    public class ValidationGroupBehavior: Behavior<View>
    {
        ...
        public void Add(ValidationBehavior validationBehavior)
        {
            _validationBehaviors.Add(validationBehavior);
        }

        public void Remove(ValidationBehavior validationBehavior)
        {
            _validationBehaviors.Remove(validationBehavior);
        }
        ...
    }
}