I have never seen a pre-shared key be automatically "forgotten" unless they didn't check the little button that says "remember this network" or something like that (assuming these are Macs).
If you enter the key correctly and check that box, when the device comes out of sleep or standy or boots fresh, it will automatically reconnect without needing to do anything.
This all assumes a preshared key network. If they are hitting a web page (captive portal), that's a whole different issue.
Go to the GUI and click on Configuration > AP Configuration > <appropriate AP group name> > Virtual APs > SSID Profiles > Guest or Tentant.
What is the authentication?