I've tried setting them up with source control (git) so that they can collaborate on my project directly, but it ends up by wasting a lot of time for me (for e.g they have windows and I develop on Mac and Linux, so it takes a lot of time to get the project running on Windows as well).

How do you do that then? Do you take the time to make your project, dependencies etc working on Windows? Do you have a staging server where designers can ftp files to? Anything else?