Invintations to group (calendar): only same domain?

Invintations to group (calendar): only same domain?

Hello,
I'm testing goup features for calendars. Using three accounts, with email addresses like:

email1@domain.com
email2@domain.com
email3@gmail.com

as a user " email1@domain.com" I created one groups and invited the other two to join.

As a user " email2@domain.com" the invitation email correctly reports a link to accept, while as a user " email3@gmail.com" the invitaion email reports a link to register to Zoho (which is not possible anyway because this user already has an account).

Seems to me that something is not working for cross-domain users, any hint?