Hi there,

There is a way to use light groups in Arnold. All you need is the ALSHADER pack.

Why do you need this?
It is simple. You want more control over the lights.
You can create maximum 8 light groups.
Each light group can contain any number of lights. You can render them in separate AOVs and adjust each light group intensity in post-production.

How to use:

• Install the ALSHADER pack (see previous post).
• Select all of your lights’ “shape” nodes (easier if you turn on the shape nodes in the Outliner). Other words: you need the light node itself, not the transform node.
• Add to them the mtoa_constant_lightGroup attribute:
  Modify menu ->  Add Attribute (Data Type: Interger, Attribute Type: Scalar)

• Set the Mtoa Constant Light Group attribute (you can choose 1-8)
  This way all lights are assigned to the corresponding light groups (1-8).
  You can find it at the Extra Attributes section.
• Go to the AOV settings -> Select the alSurface -> add the light groups to the right side.
• Check the results in the render view.

One example:
Two light groups, one light in each group. A spot light (blue) and a point light (orange).

Cheers, D