How to respawn a certain player or players?

You would have to identify that player somehow, i.e. they would have to perform an action you can capture so you can store the Agent in a custom device.
You can then respawn that Agent at a specific location.

using { /Fortnite.com/Devices }
using { /Fortnite.com/FortPlayerUtilities }
using { /Verse.org/Simulation }
using { /UnrealEngine.com/Temporary/SpatialMath }

player_respawner_device := class(creative_device):
    # For example, pressing a button will be the action that registers a player to be respawned
    # If could be anything that callbacks with an Agent
    @editable
    Button:button_device = button_device {}
    
    # The transform to respawn the player at
    # Alternatively, you could reference an in-world prop/device and call GetTransform() on it
    @editable
    Target:transform = transform {}

    var RegisteredPlayer:?player = false

    OnBegin<override>()<suspends>:void=
        Button.InteractedWithEvent.Subscribe(RegisterPlayer)

    RegisterPlayer(Agent:agent):void=
        if(Player := player[Agent]):
            # Here we store the last player that interacted with the button
            set RegisteredPlayer = option {Player}

    # Call this in your code when you want the player to respawn, or register it as a callback for a specific action
    RespawnRegisteredPlayer():void=
        if(PlayerToRespawn := RegisteredPlayer?):
            PlayerToRespawn.Respawn(Target.Translation, Target.Rotation)

I hope that’s what you were after?