Start with Mac, find some good metrics that indicate that a lot of your users are on Mac and go from there. (parse your user agents for instance) Once it's proven itself, you can try Windows and some other options as needed (probably mobile).
We're talking about a chat application for diverse corporate teams. If you don't have native apps for Windows/MacOS/Android/iOS at the get-go then you've missed the boat.
If you build it, they will come. Especially if they don't have a choice. So build the new, deprecate the old.