I was looking for the same information. Apparently there is a banner that will pop up in the members app to sign up. On my Note 8 I was part of the One UI 1.0 beta and we signed up under the old Samsung app. Not a big fan of the new members app but getting used to it.
In order to join the note 9 beta program you must open the samsung members app on that phone, go to your notifications, then click register for the beta program. After, go to settings and check for updates. Then install