Google Sync 

Here are some of the main things you need to remember to do:

1. Make sure you have enabled Google sync for mobiles in your Google Apps account. From your Google Apps Dashboard, click the Mobile link. Then tick the Enable Google Sync box on the next page and press save.
2. Set up Google Sync on your iPhone to sync calendars and contacts from Google. Basically you just add a Microsoft Exchange account with your google apps login details, and then select calendars and contacts in the syncing options.
3. You must also choose which calendars you want to sync to your iPhone via the Google Mobile website. Visit http://m.google.com in Safari on your iPhone.

iCal

You can set up iCal to subscribe to your Google Calendars via a caldav url. First you add a new account under preferences, and then under the delegation tab you select the calendars you wish to subscribe to.

Sharing Google Calendars 

You can share a calendar with other users in your google apps domain, or external users on other domains. There are various ways to do this such as sending a link to the ical url or sending an invite within the calendar sharing pages on the Google Calendar website.