It may also be worth a ping to @gassim and @UnicornX as well to see if this has solved it for their use cases, and round out those topics too. 
I’m getting an ‘invite expired’ on the one in Join the Discourse User Testing Community, which is encouraging as that sounds like it would work again if the invite redemption was extended. (Though that might also be because I already used it ages ago and forgot about it 
)