Wow, this change is just awful and the response from Google has been even worse. I echo everything already said in this thread, and add to that the fact that the "error" message shown when trying to add an event invitation sent to a different account isn't even accurate in all cases. It says:
"Google Calendar invitations cannot be forwarded via email. This event
belongs to m...@mydomain.com and you are logged in as m...@gmail.com. Please ask the meeting organizer to add you to the
event from Google Calendar."
But I did not receive the invitation via a forwarded email. I read it in my "alternate" account's email reader and tried to click on it, thus opening it in my browser. The message is thus nonsensical.
The sentence "Please ask the meeting organizer to add you to the
event from Google Calendar" is also unclear. The meeting organizer already invited me via my other address.
Of course fixing the language of the "error" message wouldn't solve the true problem -- broken functionality.