If you have Hue lighting, select the Hue Rooms under "Select Bulbs for Group". This is the same place you would select bulbs directly connected to the HE hub.
If you're dealing with directly connected bulbs, you may be able to eliminate the popcorn effect by first adding the bulbs to a Group and then turn on the Zigbee Group Messaging option. If your bulbs support Zigbee Multicasting, this may help eliminate the issue. Then add the Hubitat Group to your Hubitat Scene under "Select Bulbs for Scene".
NOTE: The Zigbee group messaging option is only for directly connected bulbs, not hue bulbs, since the Hue Bridge already supports that when you create Rooms in the Hue app. Some bulbs simply don't support Zigbee multicasting and this option may have no effect.
I'm not 100% positive, but it might be an issue if you combine the Zigbee group messaging option with Hue Rooms, so in that case I would make a separate group for any directly connected bulbs with that option enabled and combine Hubitat Groups and Hue Rooms together in your Hubitat Scene.
However, I could be wrong about that and it many not be an issue to have Zigbee Group Messaging enabled when Hue Rooms are also selected. @bravenel can you weigh in please with the correct answer?