Fill out this form to download the beta version of Family Diagram.
While the beta diagram is fully functional and currently offered free of charge, the app will only be available on a paid subscription basis once it is released. A subscription will cost in the range of $150-$200/year and will support the significant cost of ongoing development.
From that point on you will not be able to use the diagrams you create without buying a subscription to use the app. However, anyone will be able to download and use the app for a single diagram. It is unclear at this point whether it will be possible to important an existing diagram into that one free diagram.
The beta currently only runs on macOS. The oldest version of macOS that is supported is High Sierra.