There are ways to get an estimate of people's lat/long based off their IP. However, converting that lat/long for a spherical Earth to a point on a rectangular map is very complicated. If you really want something like this, you are probably better off using a database and Google Maps.

1) Users are logged to the database or file, and their IP is converted to lat/long up front using some sort of tool.

2) AJAX is used to query the database for new entries since the last check (pass the most recent sequential ID that you have, database gives you everything after that)

3) The entries are dynamically added to the Google Map using their API.