As we all know, you are not allowed to ask question in the Apple Support Community if it's related to a beta. The mods delete it immediately. That being said, I'm having the same issue. In beta 1 and beta 2. I can log into 2 devices, but the 3rd and 4th fail. I tried different devices this time, but after the 2nd login it starts failing on the last 2. I think they put some limit on it.

Very, very gradually. Seems like only a very few got access, the rest are in a holding pattern. Apple still hasn't fixed Universal Control and that is going on 3 years. I have little hope for Image Playground, it will be something Apple announces and then forgets and lets languish. Doesn't really matter as there are a ton of much better AI image generation tools out there. I don't really have a use case for generating cartoons and gen-moji, whatever that is.... sounds gimmicky.