Sign in with an e-mail code and accept an invitation

Only invited people can sign in. There is no password and no self-registration: an owner or admin first invites your e-mail address (see the page on members and roles), and only that address works.
On the sign-in page type your e-mail address and choose Send code. An e-mail "Your sign-in code for marsstein.ai" arrives from the sender name accounting-ai with a 6-digit code. The code works once and for a few minutes only (the e-mail says how many). Type it on the screen Enter the code and choose Sign in.
The page says "a code is on its way" for every address, invited or not, on purpose. If nothing arrives, check the spam folder and the spelling, then choose Send the code again (only the newest code works) or Use a different email. If it still does not arrive, the address most likely has no invitation: ask the person who manages your account to invite you.
Code problems: That code is not correct means a typo, so type it again. That code has expired and Your sign-in has expired mean you need a new code. Too many incorrect codes. This code no longer works appears after several wrong tries: choose Send the code again. There is no invitation for this email address means the invitation is missing, withdrawn or expired.
Codes are limited per address and per hour (5 per address by default). Beyond that the page says Too many code requests and names the minutes to wait. Wait that long instead of clicking again.
An invitation e-mail "{person} invited you to {firm}" has the button Accept invitation and a valid-until date (14 days after it was sent). The button opens a page with the firm and your role. If you are not signed in, choose Sign in to accept and sign in with the invited address: the invitation is then accepted at once. If you are signed in, choose Accept invitation; you see "Invitation accepted" and land on the firm's clients.
Invitation errors: This invitation has expired or has already been used and This invitation was withdrawn: ask the person who invited you for a new one. This invitation was issued to a different email address: sign in with the address the e-mail was sent to. The person who invited you may no longer invite: ask for a new invitation. If a screen says Your session has expired, choose Sign in again.