Hi HardwareMonkey,
I'm not sure if there is a way of limiting the number of users, but a lack of bandwidth would certainly be a limiter.
I once connected my laptop via Bluetooth to my mobile phone which then dialled the internet on GPRS. I tried to connect to my Broadband public IP address which connected to Broadwave on my desktop PC.
The link speed was so slow that most of the time the player (Winamp) showed "buffering" and played a few seconds before buffering again.