How to make an Infared Player View? How to go about it?

So, I haven't found anything about this regarding help but I've seen other games do it. I'd like to know how to make an infrared/x-ray view of players. What I mean is like being able to see where players are by putting a dot on them that can be seen through walls locally to the player.

Some things I've tried was a billboard gui but ran into the problem that it wasn't local and it would show it for all players. Basically, I want a player to press 'u' and it would put a dot on every single player in the game. The dot can be seen through walls and they can follow it. Just a way of finding players on a map.