Check the code
Use the current eight-character code and check for typing mistakes. Ask the owner to copy it again if needed. Share it only privately.
Check the message
Invalid code means the app could not find a matching household. Household is full means its current member limit has been reached. The owner should review the current plan and membership.
Do not keep retrying blindly
If the connection failed, refresh Household to see whether you joined. Contact support if the membership state remains unclear.