Is it possible to communicate to other WiFi users in the house?

Solution 1:

The only way that I can see is using a computer behind the Wifi-router configured as proxy of your network.

Besides the fact that your network sharers will need to log-in onto the proxy to enable network access, you can create a "broadcasting" message blocking traffic for some time and spreading a specific web pop-up that will broadcast any message that you want for all your users. I never did such thing but I think using a proxy is the best scenario for you if you are sharing your network bandswitch over Wi-fi.

Solution 2:

Generally, just because you use the same WiFi access point, it doesn't mean you'll be able to communicate with them.

Your ability to communicate with them is largely dependent on what operating system the other users use and what communication tools they have installed/available.