Usability for invited guests (same or other email)

Microsoft has struggled with this and its seriously time for you to sort this out.

It is not easy joining an external channel with your same login or being a member of another team in another organisation using the same email, or a different email accessing a different account.
All of these are valid use cases.

In my instance, I would like to see a channel in my Teams that belongs to another organisation. 
I wish them to see our channel in their organisational list of channels as an "external channel".

Please allow us to invite others and allow them to access the channel seamlessly. It is ridiculous that this cannot be done. And no one really cares about your admin/office365/windows user accounts/ nonsensical "account" problems because you have deliberately taken this approach and its wrong.

Everything should work on whether you have permission in a channel - not an org.

I trust you get this right soon, because its a massive stumbling block and a source of great agony to all involved.