You might be able to log into the AP's web interface and find a list of associated MAC addresses.
If it doesn't tell you the IP of them, you can try arping which should tell you the IP.
It may be possible to sniff the channel the AP is on if your WLAN card supports monitor mode - you might be able to enumerate any MAC you see on the network at the very least. I know there's tools for Linux that might accomplish this, don't know about Windows. I don't have any experience doing this but may be something to look into.
Your AP may have "AP Isolation" enabled which prevents you from contacting any other STA on that WLAN.