Set up user roles that you will assign to users in order to authorize them to access applications and services hosted on your account. User roles correspond to the activities performed by users. When you create a user account, you are prompted to select the required user role. There are several predefined user roles. You can review and modify them, or you can create your own custom roles.

Implement predefined "User role" on existing user of domain.

Login into Plesk control Panel and go for the "User" tab and then select the user on which you want to set the role.

Now select the desired "User role"(predefined) for your domain user from the drop down box and click OK.   

New "User role" implemented on the User.

Creating a modified User role.

There are some predefined user roles but you can create your own custom user role as you wish for your multiple users.

Like previous steps at first go to "Users" tab then select "User Roles" tab and click on "Create User Role".

Now write the User role "name" in the box, select the services you want to permit the user and click "OK" to save it.

A new user role is created. Whenever you want to select the "User role" for a user it will be automatically shown on that drop down box from where you have to select the desired "User role".  

To select that custom User role you have to follow the same steps as Implement predefined User role on existing user of domain.