I tried it. You *HAVE* to have a Google account to sign up.
I'm sorry about that. My understanding is that those who are invited
do not need a Google account to sign up, but the information on the
internet about this is sketchy.
I have been sending invitations but there's a daily limit of 500 and I
haven't got to the email addresses starting with S. I'll try it
tomorrow.
Is there really no other option?
We did not find a convenient one. These days, setting up a mail server
that Gmail finds trustable is not trivial. Moreover, over half the
current subscribers have gmail addresses.