I don't think you can do it from iCal, but you can establish a Google account (if you don't have one) and do it directly from Google. Google explains how here:
Basically, you go to "Settings" and click on "Mail, Contacts, Calendar." The first section is "Accounts." You can set up a Gmail account, similar to an iCloud account, and then decide whether Google Mail, Calendars, and/or Notes should be synced with the appropriate app on the iPhone.
I've had my Gmail synced to my iPhone for ages but only turned on Calendars this morning. Worked like a charm -- I have many Google calendars, mostly mine but some that have been shared with me, and they're all there.