Superusers are administrative users within a team, and have full access to all of CaptionHub's functionality inside of that team.
Producers are administrative users within a project. They have to be assigned to specific projects, by a Superuser. Within a project they are assigned to, they can add and remove users from caption sets, invite users to the project, hand caption sets between users, and download completed files. They cannot translate caption sets.
Optionally, Producers may be given the power to create projects. In such cases, they'd automatically be assigned to those projects as a producer.
Linguists can translate language versions. They belong to a team, and are marked as having competency in specific languages. They can only be assigned to language versions that are part of projects in their team, and that they have proficiency in the language of. Unless a linguist is specifically assigned to a language version, they won't be able to see it.
Validators are a role that can be enabled in Team Settings by disabling the ability for linguists to approve their own work. Optionally, Validators can have edit permissions disabled – so they'd only be able to approve, not edit.
Users may belong to more than one team – they may, for instance, be freelance. A user can only see users that belong to teams they do, and they will not be able to see if they're a member of other teams. If you see that a user is in more than one team, it is because you are in those teams as well.