Short answer: Most login and invite problems come down to signing in with a different email address than the one your invite was sent to, or an invite link that has already been used or replaced.
If you can't sign in
You need to sign in with the exact email address your invite was sent to. If you sign in with any other account, even one you use for other work, you'll see a message telling you the invitation was sent to a different email.
If you already belong to another organisation under a different sign-in, you won't be able to accept a new invite on top of it. Each person can only belong to one organisation at a time.
Check the email address you're signed in with matches the invite exactly.
If it doesn't, sign out and sign back in with the correct account.
If you're still stuck, contact whoever sent you the invite (usually the firm owner) and ask them to confirm the email address they used.
Note: Email is fixed to your signed-in account throughout onboarding. There's no way to change or re-enter it once you're in the portal.
If your invite link has expired
Invite links are single-use and time-limited. A link can stop working for a few reasons:
It's already been used to accept the invite.
Too much time has passed since it was sent.
The firm owner sent you a newer invite, which automatically replaces the older one.
If your link no longer works, the fix is a new invite, not a fixed link.
Let the firm owner know the link isn't working.
Ask them to resend your invite from the delegate panel.
Use the new link as soon as you can, since it will also expire or be replaced if another one is issued later.
Tip: Invite emails are sent on a best-effort basis and can occasionally fail to arrive. If you never received a link at all, ask the owner to check for a warning about the email not sending. They can copy the link directly and send it to you another way.