Hello @Tom_Griffin sorry to hear about the issues you are facing.
If all seems good from your end and no typos in the email and the invite cannot be found in their junk folder I recommend asking them to go to asana.com and upon trying to login they can just click „forgot password“. Then they will receive an email to reset the password and should be able to login.
I had this situation once as well and the password reset did the trick.
Otherwise definitely Asana support will definitely be able to assist.
It is weird that you have not received a response from them yet, I recommend sending another message.
I hope that helps 