LightGroupManager

public void AddRemoveLightGroupInManager(Document document, FamilyInstance lightOne, FamilyInstance lightTwo)
{
    if (document.IsFamilyDocument)   // it supports project document only.
        return;
    LightGroupManager groupMgr = LightGroupManager.GetLightGroupManager(document);

    // Add a light group with a light
    LightGroup lightGroup = groupMgr.CreateGroup("Group_One");
    lightGroup.AddLight(lightOne.Id);
    // Add another light group with another light.
    lightGroup = groupMgr.CreateGroup("Group_Two");
    lightGroup.AddLight(lightTwo.Id);

    // Retrieve the added light group in the manager.
    IList<LightGroup> existingGroups = groupMgr.GetGroups();
    foreach (LightGroup group in existingGroups)
    {
        string groupName = group.Name;
    }

    // Remove one light group from the manager.
    groupMgr.DeleteGroup(existingGroups[0].Id);
}