If you have an Apple Mac then below are instructions from Apple on how to had an Internet Calendar (.ics) to iCal.
(If you've not yet set up the .ics link check out this guide first.)
Subscribe to a calendar
To subscribe to a calendar from a link on the Internet or an email you received, click the link. If you do this, you can skip step 1, and the calendar’s web address is filled out for you in step 2.
In the Calendar app 📅 on your Mac, choose File > New Calendar Subscription.
Enter the calendar’s web address, then click Subscribe.
Enter a name for the calendar in the Name field, then click the adjacent pop-up menu and choose a colour.
Click the Location pop-up menu, then choose an account for the subscription.If you choose your iCloud account, the calendar is available on all your computers and devices that are set up with iCloud. If you choose On My Mac, the calendar is saved on your computer.
To get the calendar’s event attachments or alerts, deselect the appropriate Remove tickboxes.
Click the Auto-refresh pop-up menu, then choose how often to update the calendar.
To prevent alerts from appearing for this calendar, select “Ignore alerts”.
Click OK.To make changes later, click the name of the calendar, then choose Edit > Get Info.
Unsubscribe from a calendar
If you want to stop subscribing to someone else’s calendar, you can unsubscribe from it.
In the Calendar app on your Mac, Control-click the calendar in the calendar list, then choose Unsubscribe.If you don’t see the calendar list on the left, choose View > Show Calendar List.
Please note this advice has been taken from Apples help pages
How do my people generate their own .ics link?
If you're letting your people manager their own calendars they can simply generate the .ics links themselves.
Here is how you do it
Calendar > Click on the calendar icon > click 'generate'