According to Apple, “You can now invite people to try out beta versions of your Mac apps and provide valuable feedback before release. Invite up to 10,000 external testers using their email address or by sharing a public link, create multiple groups of internal testers and configure build access for each one, and more.”
Prior to this, TestFlight beta 3.2 for macOS was only available on the Apple Developer website, but with the release of TestFlight 3.2.1, anyone can install it. However, keep in mind that if users want to take part in betas for the Mac, they will need to have at least macOS Monterey installed on their computers.
This means that users with older Mac computers that aren’t eligible for the update won’t be able to take part, plus with the various issues surrounding macOS Monterey, we’re pretty sure that many users are also holding back on updating until Apple sorts all the issues out.