The person whose calendar you want to share needs to right-click their calendar and choose properties, then go to the permissions tab and add you in as a reviewer or whatever permission level they want you to have. I do not recommend they add you as a delegate. That causes too many issues should your account become disabled in the future and they forget to remove you as a delegate. Delegates should only be added for someone you are giving COMPLETE control of your calendar. Meaning they will accept/decline and set up meetings all on your behalf. Otherwise just grant permissions to the calendar itself.