At this time, there isn't a cross-platform Google Contacts mobile app. iOS users could use the native Contact app (see Use Google Apps on iOS), Android 2.1 - 2.3x users could use Google Apps Lookup.
End-Users can access the Global Directory by using Gmail or Google Contacts desktop web apps. They could try to use these apps from the mobile web browser, but at this time the Google Contacts web UI is not responsive, in other words, could be hard to use it in a small touch screen.
If users only want to access people on the same organization and Google+ is enable for the organization and the users, they could try to use Google+ mobile app. An alternative that could work for people from a small business is to move the Directory contacts to "My Contacts" by using a desktop browser in advance of trying to search for contacts on the users mobile devices.
A hack that you could try is to a add your Google Apps for Work account as it was an Microsoft Exchange account by following a similar setup the resulting one by following the directions given by Google on Sync Calendar, Gmail, and other Google Apps with your iPhone