Unicorns! Get your fresh Unicorns!
From the articles defining the Board....
The European Data Protection Board (the ‘Board’) is hereby established as a body of the Union and shall have legal personality.
The Board shall be represented by its Chair.
The Board shall be composed of the head of one supervisory authority of each Member State and of the European Data Protection Supervisor, or their respective representatives.
This is going to be no different to Euratom, Europol etc
If you're not in the EU, you're automatically not in the agencies whose membership is drawn form EU states.