startup.lua

function main()
    local chatBox = peripheral.find("chatBox")
    local playerDetector = peripheral.find("playerDetector")

    rednet.open("top")

    while true do
        local playerToExpose = listenForRequest()

        local onlinePlayers = playerDetector.getOnlinePlayers()

        local foundPlayer = false
        for i, name in ipairs(onlinePlayers) do
            if name == playerToExpose then
                foundPlayer = true
            end
        end

        if foundPlayer then
            local playerPosition = playerDetector.getPlayerPos(playerToExpose)
            local message = "This you, "..playerToExpose..": "..
                            playerPosition.x.." "..
                            playerPosition.y.." "..
                            playerPosition.z.."?"

            chatBox.sendMessage(message)
        end
    end
end

function listenForRequest()
    _, playerName = rednet.receive("expose")
    return playerName
end

main()