Tell them the auditors are looking at _processes_ and they should not take things personally. That if problems are reported, everyone will be invited to participate in civilised root cause analysis and corrective action. There will not a witch hunt or a blame game. For example, you might say, if people are doing the wrong things through lack of training, the training will be provided and the process for making sure people get it when they need it will be improved.
If you can, get the most senior manager available to say this, in public, at the opening meeting, to help develop the trust necessary for a realistic, honest appraisal of your system. (Of course it takes more than that to establish trust, but it's a start.)
But, if blame games are common in your neck of the woods, you might just want to tell them to take a vacation ... or, more seriously, in your role as "guide" make sure the auditors meet the difficult managers and write findings in section 5, on management responsibility, instead of fueling blame games by picking on the little guys who can't change anything. Not that that ever happens, of course ...
On the other hand, if yours is a civilised organisation where managers actually manage (and they do exist), please ignore this post, you'll be doing it already.
Patrick