Exempt users to watch YouTube

Morning Guys.

I am new to this Fortigate thing. I am using Fortigate 300D with OS 5.4.3 build 1111.

I want to add few users to a group that i will allow to have YouTube all day and block the rest of the users from accessing it. I know i am expected to create a user or user group then later add it to schedule and create an IPv4 policy for this. I am stuck where it needs me to create users and passwords for these guys. Please note that i run through a proxy, my authentication is on windows not RADIUS or any how based on the Fortigate. My concern is what password are these because i cannot have their passwords myself, or do i create any passwords for them and hope it will not interfere with their domain passwords?

I have blocked all streaming sites so these users i need to exempt on YouTube often ask me to open it for them for a few hours so they check something out. i just want to create a schedule for them to access it all day.

My question is how do i go about it to add about 6 users to be exempted from YouTube? A step by step guide will be good.

Thank you