I have this character swapping and I would like to put a blue circle around the bottom of the current player (on the ground) but honestly have no idea how I would go about doing that, any help would be great.
Hello,
I would add a decal (like in the top down template) and set it visible on swaping your active camera or player (don’t knows how you managed the swap 
).
Hope this help.
This is probably a hack way, but you could always put a circle under the character in the character BP as a plane or mesh and just set it to visible or not depending if that character is the one being controlled.
Have you considered a deferred decal?
You just would need to add one as a component to the actor’s blueprint and it would then cast down on the ground around the player. You would just need to create a decal material with the texture you wanted.
Problem at Decals is:
It will always draw on other Objects, like Characters that stand near to each other, too.
Putting a Circle in the Blueprint, as mentioned by Jimminy_Cricket, is the easiest way.
There is a tickbox that enables/disables whteher Decals are rendered on the mesh or not.
I’d just have it as near the ground as possible, potentially with a height check for un-equal grounding. If it’s a ring shape then the player model would be inside the alpha’d area anyway.
Many ways to skin a cat.