Creating a panel is an integral step for reviewing configuration (Reviewing > Panels. Read more about panels here). When defining who will review on a particular panel in the 'Who' section of the panel configuration, there are two options available: 'any users with a role' or 'invited reviewers'.
Selecting the first will display a list of all the roles available in your account and anyone with the selected role(s) will then be on the panel.
Selecting Invited reviewers will allow you to search for specific users by typing in their name or using the Advanced search to filter by a role. Note that this will only include users in the account who have a role with the Scores (own) permissions set to Allow. By default, the reviewer and grant manager roles have this permission already.
It is possible that other roles have been created or the permission settings adjusted for existing roles that require the Scores (own) permissions to be set to Allow. Before adjusting the settings to prevent such users from appearing in the panel list, ensure that doing so will not inadvertently disrupt another function of the role.
For more information on user roles and permissions, see our guide here.