I’m a bit rusty on Google calendar but I was using it a lot, not sharing calendars though.
From memory should be ok.
If the freelancer has a Google acc, they should be able to create their own and invite you specifically to share, you can then pull as many of those as you want into a non published calendar of your own.
I used to pull in several calendars from project management apps like “Insightly”, so linking people should work ok.
If you’re using Google Apps it could all be done inside the one account by registering users, as Admin you can decide who see’s what, I was using Apps for Business (paid model) so each user was $50 per year.
Someone using the free version might have a better idea.
A lot of the project management web apps have this kind of facility built in, Insightly for one, Mavenlik I’ve used and have just started to set it up again, could be overkill if you only need availability but if you need to upload files or collaborate as well, they might work, they integrate with Google account too (the better ones).