If I have a private smart contract that I want to invite a new member of my consortium to, can I expand the scope of the private contract to provide them access to the full 'private' ledger?
No - the nature of private transactions on Ethereum doesn't allow for members to retroactively access the state of a private smart contract to which they were not a member at the time of the transaction(s). When private transactions are executed, they are not encrypted with a common key shared amongst members but rather they are individually encrypted and delivered to each of the addresses included in the privateFor parameter at the time of the transaction, with the ensuing state independently stored in the nodes' private databases. This means that a new participant in a private smart contract would not be privy to any historical transactions, as it would require one of the private keys of the individual addresses to which the transaction was originally scoped.