Well that is very odd, since even machines on a LAN are connected to the Internet. That said, as the normal 'end user' doesn't know what a IP stands for much less how to find it, trying to connect the application between one user to another relied on either external use (Yahoo, Skype, etc.) making up 'fake' accounts for each team member or relying on including collaboration / groupware software has been the norm for quite a long while now.
Using a simple Google there are some offerings, but as noted, only a couple because this sort of request is a throw back more then a decade that no one 'does' anymore. You could try
http
/lanmsngr.sourceforge.net/features.php or one of these...