In order to join non-domain devices to your network, you have to achieve one of the following:
1. Supply the certificate used for authentication and install it in the "trusted root certification authorities" store if it's a windows device.
2. Uncheck the box that says "Validate Server Certificate" in the PEAP settings... Make sure you're using Microsoft: Protected EAP(PEAP) and not "smartcard or other certificate".
Thanks,
Tony