If you don't have access to a Mac, but want to build for Mac, one option I have noticed is that you can rent time on a cloud based Mac. A quick internet search should reveal several options for you.
If you purchased the Desktop license, this permits you to run GMS 2 on both PC and Mac. (The Creators license only permits you to build on the platform you purchased it for).
The project files are compatible between PC and Mac. If you have created your project on PC, you can just copy across your files, open them in GMS 2 on a Mac and it should run just fine. There are some different setup steps however if you are wanting to create a YYC build via Xcode, as well as needing an Apple Developers license to build for Mac App Store etc...
About sharing your project files with someone who has a Mac. If they have their own Desktop GMS 2 license, this is probably ok as you would have both purchased a copy of the appropriate license, and they could make a build for you. But to be 100% sure you should check the license agreement. But this should be a normal use-case for people working in a development team as well, so as I said, as long as you both have copies of the Desktop license, it should be ok.
All the info above is available in more detail if you check the Yoyo helpdesk website, there are many guides and faqs about setting up builds for Mac etc... Link :
https://help.yoyogames.com/hc/en-us