There are three common causes when a teammate you invited can't log in.
First, they may not have accepted the invite yet. An invite email is sent to the address you entered, and the teammate needs to click through and accept it before they can access the account — check with them that they've done this.
Second, the invite may have expired. Invites are only valid for a limited time, and if too much time has passed, they'll need a new one. Go to Settings → Team, find their pending invite, and resend it — this issues a fresh invite with a new expiration date.
Third, they may be trying to log in with a different email than the one you invited. LettrLabs matches the invite to the exact email address it was sent to (e.g. [email protected]), so logging in or registering with a different address won't work. Confirm the email they're using matches the invite exactly, and if it doesn't, cancel the old invite and resend a new one to the correct address.