Pinging a cellphone means setting up a portable cellphone tower. All cellphones within range will report their existence to you, which you can then cross reference against your missing person's IMEI number...Through triangulation/multiple different towers you can work out the location quite accurately.
Of course this is not generally available to the public. For a start you need to have a portable tower (or borrow a few from a local telco) and secondly you'll need to cross-reference his phone number to look up his IMEI.
I'm curious how a bunch of students were able to get past the two restrictions - I can imagine a uni having some portable towers lying around for research purposes, but how would they have found out his IMEI number?