Templates and plugins access levels

I am new to wordpress multisite and I am working a project that will help people:
1. create their own multisite, but I do not know how to insert a sign-up button that will help people subscribe and have their blogs created instantly.

2. Have access to themes and plugins I assign to them only (I have free and paid). I created a sample blog from the administrator side and I found out that I had access to all the enabled themes and plugins for the network even the admin theme.