Squarespace Website – Admin controlled users to members area

Hi! Firstly this is a short term project, if you can help me do this quickly and effectively and have the right skills, I will be looking to work with you for the longer term.

I have developed a squarespace website for my business which gives the ability for clients to book appointments.
This is the problem I need solved is, I would like to to restrict access to the bookings page.
I know this can be done using the members area, however I don't want anyone to have the ability to create their own account by "signing up". If I choose to work with somebody, I want the ability to then provide them with a username and password to log in to be able to book appointments.
Ideally looking for someone to help solve this issue quickly and someone who already has a possible solution in mind.

If you have other experience setting up automations using Zapier or other automation tools and setting up CMS platform set up this would be very desirable for the longer terms projects. (will be willing to renegotiate contract for this).

Look forward to meeting you!