The most common problem in adding a Secondary Parent/Guardian is if their email address is already in use.
This can happen if the Secondary Parent/Guardian previously had their own Rooster Money account, or they may have accidentally created their own Rooster Money account instead of joining an existing family.
The fix this yourself, the Secondary should first try and log into the account that already exists with their email address (you can use the forgotten password link if you no longer remember your password).
Delete the duplicate account from within the app by tapping Menu > Manage Account > Family Account Settings > Delete family account.
The Primary parent can then send over a new invite email to get the Secondary set up on the family account.