I am wondering if there is a way to send a message to users that have entered your wifi's reach? I am not sure if this is possible or how it's called.
A hint of what to look for or some sample code (if this is possible at all) would be much appreciated!
The only thing that is possible as far as I know is to add software to your accesspoint to do a splash screen (or possibly a login page).
This would not completely solve your use-case, but like the various "free" wifi in bars you do need to log in to first, you need to open a browser first and perform an action, before you have a real connection.
You could just skip the authentication-part, and put your message on the splash screen.
A random example of what you can find on the internet is this: http://www.dd-wrt.com/wiki/index.php/NoCatSplash I have no experience with it, but it seems to serve above mentioned purpose, and you can easily go from there.
I am aware that this is NOT an android specifi answer, but as far as I know it is not an android specific question: you want to send a message to people connection to your wireless.