The open beta is available to eligible devices, each time it comes around. The eligible devices however can be quite tricky sometimes to know, but typically it will be devices bought directly from Samsung without any carrier software from companies like EE, O2, Three, Vodafone etc. You'll know if your device is eligible as it will have a banner in the members app inviting you to join the beta. It'll also be in your members app notices, typically where I always notice it 😂