Enterprise Account Management

So... Let's say you have a client with over 100,000 employees with a lot of turnover, and that the on boarding is handled via a third party.  The user population could reach 150,000 due to spouses being included, and was imported to the previous site via a delimited file through SSIS.  What would be the recommended/best practice for setting up a fully automated user import process including role assignment?

I know that it could be different for everyone depending on what data needs to be included, but I am just wondering what overall methodology would be used.  Would a custom application have to be written or could you use SSIS?