Create a Staff Member record for the subcontractor under Settings > Staff Members and set their permission level to "No Access".
You can then assign Jobs to them and schedule appointments accordingly, however, they cannot access the system in any way. Essentially they record their time and other costs manually outside the system, and these are either entered on the Job by a User that has access to Tradify.
There is a small charge for No Access Staff Members. For more information on the pricing, please click here.