Using plugin is extremely easy - you just have to open Roles tab on Project view page.

We will go step-by-step and explain every action and each interface element.

Main page

Open your main page and click on "Projects".

1. Default Jira Projects tab.

Projects Tab

1. Choose your project

Project homepage

Click on "Roles" button to open Roles tab view.

1. This button navigate to project role tab view

Role tab view

Users are grouped by their role in the project. User name are clickable and lead to public profile page. 
User Avatars were added to make distinguishing easier.

1. Roles - as defined in System → Security → Project Roles
2. Users - as defined in Project Settings → Users and Roles.