How can I sink employee ical calendars to one calendar?

It's not really clear what you mean, but I'll venture a guess.

Back up all data. If you synchronize with iCloud, back up your iCloud calendars too.

Select one of the calendars from which you want to merge events in the calendar list. From the menu bar, select

File ▹ Export ▹ Export…

You'll get an ICS file with the filename extension ".ics". Delete the selected calendar by right-clicking it in the list and selecting Delete from the contextual menu.

Double-click the ICS file you just created. Import the events from into the desired calendar. If everything looks right, delete the ICS file and go on to the next calendar.

If you don't have your own calendar server, and you want a true multi-user shared calendar, then your needs are fairly complex and you'll need a hosted service. I don't have a specific recommendation, but Google can probably do what you want.