In my opinion this looks more like a problem with your PC rather than the game. My hypothesis would be that your NIC stops responding once it has to upload too much bandwidth to the other connected players on your server. As to what causes this to break WLAN would be interesting to find out.
Couple of things I’m curious about, how many people have to join until it stops working? Did this start happening just after the 0.1.5.1 update? No changes have been made to the PC at the same time like system configurations of any kind, or any recent Windows 7 updates/drivers?
Strange. If you open up Event Viewer on Windows, can you find any errors that pinpoint to the time when WLAN stops working? I am not exactly sure if Windows event logs will show anything useful, but it might show something about the WLAN service if you’re lucky.