Mongrel Permissions and Cache

We've just started to implement page aching for our app, and it all works fine on our development machines, but when we move to staging there's a problem, the cached don't get written into the public directory because Mongrel don't have the right permissions (we're using Apache 2 and Mongrel Cluster). In fact it throws an error. I'm not much of a sys admin so I can't work out how to find out which groups the Mongrels belong to and how to add them to a group that does have permissions. Any help here would be greatly appreciated