It's a fair and very important question. @Rustler brings up a key point - by definition, a sub-contractor cannot be part of your clients' business. There's been a lot of change here in California about what defines a "contractor" in the gig economy - Money Magazine did a good piece. If there's any ambiguity, I'd talk to your client to make sure he isn't treating them as employees as this can have some very significant tax implications.
You're asking at a very good time though since tax season is so far away. Do you know to set up these contractors as Vendors in QuickBooks?