You can set one or more reminders for an event, whether or not you created the event or have permission to edit its other details.
Open the event summary and touch Details.
A new reminder is added, for your default reminder time before the event. You can change the default time; see Changing Calendar settings.
Touch the reminder time, and in the dialog that opens, touch the length of time before the event that you want to be reminded of it.
Touch Done in the Application bar.