How to get all controlled character locations?

I am working on a boss AI for what I hope will be a multiplayer game.

I would like the boss to target the closest player for a period of time, then after a period of time do another check on the closest player and switch targets to them.

Each player controls one character, but I don’t know how to get all the locations of the characters controlled by players so my behaviour tree can identify which one is closest.

Beyond that, I’m also not sure how to keep it updated in case of events like a player dying.