Yes, it's free to join. You only pay for the Apps you want to use and there are no data transport or storage fees either. All data stays on your network and remains yours, forever.
Data Historians are expensive, usually out of reach for smaller organisations. The Industrial App Store solves this with the free App Store Connect software allowing onsite storage of process data.
Do you have a cool idea for an application but don't want to get tangled up in the 'how do we get the data' minefield? The Industrial App Store is open to anyone who wants to develop industrial applications.