It depends on the security requirements. At a previous job, every user had a unique username/password to access the privileged wireless network. Guests were allowed into a highly restricted network by clicking through a "You're using our network, be good!" warning.
At my current job, we were careful to design our network so that being on the office wireless granted no privileges higher than would be granted to someone out on the internet. So there we use a single shared WPA2-PSK password that everyone knows.
I would find it hard to justify trying to keep a shared password secret. If network access needs to be protected (due to privileged server access, for example) then the only auditable and manageable system is unique credentials for each user, with good logging. If network access does not require that degree of protection, then trying to keep a shared password secret is just an annoying exercise in futility...