That's an awesome question!
Generally speaking sub-contractors are not considered Employees, but it is not always easy to establish which category an individual falls into.
If the sub-contractors in question are working on an Employment Contract with the company, then your company could contribute to an HSA on their behalf, AKA you can add them as Members on your Plan online.
However, if those sub-contractors are actually sole proprietors themselves and independently contracted, then they wouldn't qualify for an HSA with your company. They'd have to consider other personal options.
As always, if you have more questions, just say hello@blendable.ca!