Hi Sumaira,
You do not need to have Dynamics in order to assign security roles to users in CDS however, some elements of CDS and Model Driven apps still use the Dynamics interface.
There are a number of ways to get to the user administration screen, I have listed 2 ways below. Please let me know if this is what you are looking for and mark as solution if so.
From the environment with CDS click on the cog and select advanced settings (this will navigate to a Dynamics interface)
Next, select the drop-down arrow next to settings at the top and you will see a security option, select this.
This will take you to the below screen where you can create new security roles or select users and assign existing security roles.
Alternatively, another way is from the Power Apps admin portal - https://admin.powerplatform.microsoft.com/environments
Then select your environment with CDS from the list, then on the next screen on the right hand side you will see a section called access with an option to see all users. Find and click on the user you want and it will navigate to the Dynamics interface for that user where you can add a security role.
