Let say your on a opposite team and its a fps, you then see a enemy and it puts a red box around it showing its a enemy and giving its username too. How do you do it?
You can use either SurfaceGuis, BillboardGuis, or SelectionBoxes+any type of GUI for this. I recommend the third, because SelectionBoxes require the least work for the nicest effect.
Once your camera can see the enemy (you can raycast from your camera's coordinateframe to the enemy players), you can instance 5 SelectionBoxes and set the adornee to the enemy player. Of course, you want to do this in a place the enemy can't see; only the current player. The box will appear around the enemy.
You can display the username using a BillboardGui above their head, or use a ScreenGui to say something such as "Enemy (username) was spotted." or something like that. Either way, make sure that it is local to the player (inside the playergui), so that the enemy cannot see it.
You can use Billboard GUI's to do it, as screengui's are more difficult to work with.