How can Rainbow users in US collaborate with Rainbow users in China? 

I am Rainbow user in US, registered with @al-enterprise.com and sending an invite to a co-worker in China who works for a contractor company there. I send him an Invitation from Rainbow to his email id. But he cannot sign up. It seems from China, they cannot access openrainbow.com. They can only access myopenrainbow.com.cn. Since it is a different domain, we are not able to become part of one Bubble. 

So the question is, how can Rainbow users in US collaborate with Rainbow users in China? I want them to be part of my bubble so that we can do group chats and start conference.

Please advise how to achieve this.

Thanks.