By co-incidence everyone who lives in a particular house has an iphone and often does not know if other people are in the house (in terms of questions like 'is now a good time to play music/vacume/cook food?') So it would be great to be able to know what devices are currently logged on to the wifi network. Is this a possible thing?
Take a look at Find My Friends. If everyone feel's comfortable with it, you can just it to find the location of your housemates and then determine if they're at home or not. You can even use it to set notifications of when your housemates arrive home.
Otherwise, this might be a feature of your Wi-Fi router, but it will show devices on your network, not users.
If you're using an Airport, check out How do I view which devices are connected to my AirPort Extreme using AirPort Utility v6.0?:
Just click on the Airport icon and it will display a list of connected wireless clients.