You cannot choose which calendars you can access, instead the owner of that calendar will have to share it with you. See Sharing your Calendar for more details.
When other users share their calendar with you their calendar will automatically display in your list of available calendars. To view their appointments on your screen make sure you have checked the box to the left of their calendar name:
- Check the box to see the appointments on your calendar screen
- Click the down-arrow to assign a different color to the calendar.
- Drag the calendars up and down in the list to rearrange their order.