The notification will only be sent to the one you invited, which in your case is the coauthor. You can specify the email addresses in the field "People to share with" in Share dialog. If you are invited as the coauthor, but you didn't receive notification email, please ask the author to check "Send an invitation email" in Share dialog before he shares you the form. Thanks!
there is sent an email, but there's no link in for activate the account as coauthor.
Also if you sent the mail again, stille no link to accept the invitation.
There is no link at all in the email? There should be a link which sends the user to a sign in page. The person can then create a new account from the sign-in page and then confirm their email address. After that they should see the form in their dashboard. If you don't see any link on the invitation email can you please forward the email to me (firstname.lastname@example.org) so that I can see what it looks like.