As a sole proprietor you can still hire others as (sub)contractors and the only tax obligation you'll have, is to send them the 1099 at the end of the year (if, like Tess mentioned, they made more than $600). Since they're not your employees, but contractors, they are responsible for paying their own taxes.

Make sure you have agreements/contracts in place with your client and the sub-contractors too. That way everything will be spelled out and all parties will be protected.

And yes, your accountant probably has the best answers.

Good luck!