You can write (!IP) in a message and that sends your IP address (this is a feature of Plus! itself and has been for a umber of versions). For another method of making it easy on people you can have a look at the likes of
http://www.no-ip.com which has a redirection capability to redirect a subdomain that you set up to a dynamic IP address, you just need to connect to them to tell the server your new IP. I hope this is what you are after.