SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-create-subscribers

CreateSubscriberAndAddToMailingList.cs

using System;
using System.Linq;
using Telerik.Sitefinity.Modules.Newsletters;
using Telerik.Sitefinity.Newsletters.Model;

namespace Telerik.Sitefinity.Documentation.CodeSnippets.DevGuide.SitefinityEssentials.Modules.EmailCampaigns.Subscribers
{
    public partial class EmailCampaignsSnippets
    {
        public static void CreateSubscriberAndAddToMailingList(Guid subscriberId, Guid mailingListId, string firstName, string lastName, string email)
        {
            NewslettersManager manager = NewslettersManager.GetManager();
            MailingList mailingList = manager.GetMailingLists().Where(l => l.Id == mailingListId).SingleOrDefault();

            if (mailingList != null)
            {
                Subscriber subscriber = manager.CreateSubscriber(true, subscriberId);
                subscriber.FirstName = firstName;
                subscriber.LastName = lastName;
                subscriber.Email = email;

                manager.Subscribe(subscriber, mailingListId);
                manager.SaveChanges();
            }
        }
    }
}