Ask the group. If anyone is prone to seizures in a group ride, they need to let the group know...if it's been triggered before it is usually not just blinking lights that causes the condition. It would be highly irresponsible for that person to not inform the group of his condition if he is riding with them.

I ride with a friend who was injured by a roadside bomb and for some reason he now suffers from epileptic seizures from differing conditions and one of the triggers are certain patterns of blinking lights. He has to wear these bizarre sunglasses that help minimize the seizures from light flashes. He can no longer drive much less ride a bike alone. It is a challenge, but we strive to be there for him.

But seriously, if you are riding with a group...if the driver cannot see you guys on the road...there is something terribly wrong and it is not for the lack of a blinky.