You see the views tab at the top of “Users” page: “All users”, views by roles and ‘No role’. If total quantity of users is larger than sum of quantities by roles plus ‘No role’, then this option may help you to return those users back to the so called “roles field”. That is grant them some user role and check via separate view what those users are. If total quantity of users is equal to the mentioned sum of users quantity by views, then this option is not needed to you yet.
Such users with incorrect permissions record may appear as a result of incorrect import, bulk update, etc.