I recently installed update 10.9.4, which was recommended for all Mavericks users. My wifi was working fine beforehand and now won't connect to familiar networks. Ironic much? The 10.9.4 update was supposed to fix wifi problems for Mavericks!
I have tried various troubleshooting approaches and nothing works. My mac is able to see the network but not connect to it. When I try, it just says "The network _____ could not be joined" and then gives me the option to run diagnostics, which does not help. I've tried resetting the SMC and PRAM, deleting the contents of the system config folder, and of course have restarted the machine several times. I've deleted the bluetooth config file, deleted and re-added the wifi in network preferences and renewed the DHCP lease. I'm afraid I'm out of ideas.