Community roles are the highest level role you can assign. At a minimum each community has a role called 'Administrator'. This role has access to the user management area where you can add, edit or remove users.
To manage community level roles click on 'User Roles' in the left menu. Note that this page is only visible to users that already have the Administrator role in the community. If you don't see this page, ask a colleague to assign you this role.
Once on this page click the 'Select users' menu and select the user for which you want to assign the role. A list of available roles for the community will appear. Check the box next to the role you wish to assign and click 'Assign'
You can reverse this process to remove the role.