public void AddRemoveLightInGroup(LightGroup groupOne, LightGroup groupTwo, FamilyInstance lightOne, FamilyInstance lightTwo)
{
    // Add two lights into groupOne.
    groupOne.AddLight(lightOne.Id);
    groupOne.AddLight(lightTwo.Id);

    // Move a light from groupOne to groupTwo
    groupTwo.AddLight(lightOne.Id);

    // Retrieve the added lights in the group
    ICollection<ElementId> existingLightIds = groupOne.GetLights();

    // remove the light
    groupOne.RemoveLight(lightTwo.Id);
}

Public Sub AddRemoveLightInGroup(groupOne As LightGroup, groupTwo As LightGroup, lightOne As FamilyInstance, lightTwo As FamilyInstance)
   ' Add two lights into groupOne.
   groupOne.AddLight(lightOne.Id)
   groupOne.AddLight(lightTwo.Id)

   ' Move a light from groupOne to groupTwo
   groupTwo.AddLight(lightOne.Id)

   ' Retrieve the added lights in the group
   Dim existingLightIds As ICollection(Of ElementId) = groupOne.GetLights()

   ' remove the light
   groupOne.RemoveLight(lightTwo.Id)
End Sub