The Console system is geared towards individual use, so there is no sub-account feature. There will be a few layers of access in the next version, like administrator and accountant.
The only way for someone else to log into your Console now is for you to simply share with them your login credentials. This means they will have access to everything that you have access to. Plus, it would be troublesome for both you and your hired coach to schedule in the same calendar, as now the Console does not allow overlapping appointments (except for those synced from Google in a 2way sync). And you would get email notifications for each other's appointments and everything related to the clients' activities.
In the light of this, I would say it is not feasible to accommodate the other coach in your account; he would need his own Console account.