Create Groups Programmatically in SharePoint 2013

 The code below changes the built in associated groups by using the spWeb property bag directly, rather than the properties exposed by spWeb - e.g. spWeb.AssociatedGroups.Add(group), or spWeb.AssociatedOwnerGroup = group.

using Microsoft.SharePoint;
 
namespace DevHoleDemo
{
    public static class GroupsHelper
    {
        public enum AssociatedGroupTypeEnum { Owners, Members, Vistors };
 
        public static void AddGroups(SPWeb spWeb, bool copyUsersFromParent)
        {
            spWeb.BreakRoleInheritance(false);
            SPGroup owners = AddGroup(spWeb, AssociatedGroupTypeEnum.Owners, 

                             copyUsersFromParent);
            SPGroup members = AddGroup(spWeb, AssociatedGroupTypeEnum.Members, 

                             copyUsersFromParent);
            SPGroup vistors = AddGroup(spWeb, AssociatedGroupTypeEnum.Vistors, 

                             copyUsersFromParent);
            SPGroup myGroup = AddGroup(spWeb, "My Group", "An example group.", 

            null, null, null, false);
            SetAssociatedGroups(spWeb, new SPGroup[] { owners, members, vistors,

            myGroup });
        }
 
        public static void SetAssociatedGroups(SPWeb spWeb, SPGroup[] groups)
        {
            string formatString = "";
            object[] ids = new object[groups.Length];
            for (int i = 0; i < groups.Length; i++)
            {
                formatString += string.Format("{{{0}}};", i);
                ids[i] = groups[i].ID;
            }
            spWeb.Properties["vti_associategroups"] = string.Format(formatString.

            TrimEnd(new char[] 

           { ';' }), ids);
            spWeb.Properties.Update();
        }
 
        public static SPGroup AddGroup(SPWeb spWeb, AssociatedGroupTypeEnum 

        associateGroupType, 
        bool copyUsersFromParent)
        {
            switch (associateGroupType)
            {
                case AssociatedGroupTypeEnum.Owners:
                    return AddGroup(spWeb, spWeb.Name + " Owners", "Use this group to 

                    give people full control permissions to the SharePoint site: {0}", 

                   "Full Control", "vti_associateownergroup", 
                    spWeb.ParentWeb.AssociatedOwnerGroup, copyUsersFromParent);

                case AssociatedGroupTypeEnum.Members:
                    return AddGroup(spWeb, spWeb.Name + " Members", "Use this group to 

                    give people contribute permissions to the SharePoint site: {0}", 

                    "Contribute", "vti_associatemembergroup", 
                    spWeb.ParentWeb.AssociatedMemberGroup, copyUsersFromParent);

                case AssociatedGroupTypeEnum.Vistors:
                    return AddGroup(spWeb, spWeb.Name + " Vistors", "Use this group 

                    to give people read 
                    permissions to the SharePoint site: {0}", "Read", 

                   "vti_associatevisitorgroup", 
                    spWeb.ParentWeb.AssociatedVisitorGroup, copyUsersFromParent);
                default:
                    return null;
            }
        }
 
        public static SPGroup AddGroup(SPWeb spWeb, string groupName, string 

        descriptionFormatString, string roleDefinitionName,string associatedGroupName,

        SPGroup parentAssociatedGroup, bool 
        copyUsersFromParent)
        {
            SPGroup owner = parentAssociatedGroup;
            if (associatedGroupName != "vti_associateownergroup")
            owner = spWeb.SiteGroups.GetByID(int.Parse(spWeb.Properties

                         ["vti_associateownergroup"]));
            spWeb.SiteGroups.Add(groupName, owner, null, 

                                 string.Format(descriptionFormatString, 

            spWeb.Name));
            SPGroup group = spWeb.SiteGroups[groupName];
            if (descriptionFormatString.IndexOf("{0}") != -1)
            {
                SPListItem item = spWeb.SiteUserInfoList.GetItemById(group.ID);
                item["Notes"] = string.Format(descriptionFormatString, 
                string.Format("<a href=\"{0}\">{1}</a>", spWeb.Url, spWeb.Name));
                item.Update();
            }
            if (roleDefinitionName != null)
            {
                SPRoleAssignment roleAssignment = new SPRoleAssignment(group);
                SPRoleDefinition roleDefinition = spWeb.RoleDefinitions

                [roleDefinitionName];
                roleAssignment.RoleDefinitionBindings.Add(roleDefinition);
                spWeb.RoleAssignments.Add(roleAssignment);
            }
            if (copyUsersFromParent && parentAssociatedGroup != null)
                foreach (SPUser user in parentAssociatedGroup.Users)
                    group.AddUser(user);
            if (associatedGroupName != null)
            {
                spWeb.Properties[associatedGroupName] = group.ID.ToString();
                spWeb.Properties.Update();
            }
            spWeb.Update();
            return group;
        }
    }
}