A parent says the code does not work
The four causes, in the order they actually occur.
Operators"Invalid or expired code" has a short list of causes.
1. It is the wrong code
The commonest by far. Each team has its own code, and a parent handed two codes will use the wrong one. Confirm which team they should be on, then read them that code.
2. It was mistyped
Codes are short, but a zero and an O look alike when read off a screen. Ask them to scan the QR instead — that path has nothing to type.
3. They are already a member
"You're already a member of this team" is not a failure. They are in; the app is telling them there is nothing to do.
4. The code was reissued
If you had a code changed, everyone holding the old one gets rejected. Send the new one.
Tip: When a parent is stuck, invite their email address instead of debugging the code. The invitation carries the code as a link and skips typing entirely.