It is very easy with OctoberCMS relationships. I think you have defined the relationship in your User
Model, if not defined it first like below,
public $belongsTo = [
'catgeory' => 'YourPluginName\YourAuthorName\Models\Category'
];
In place of YourPluginName & YourAuthorName, please define your author and plugin name.
After defining the relationship, you can retrieve user's category by $user->category
e.g. Suppose you have the first User then you can retrieve category by,
$user = User::first();
$category = $user->category;
//and now you can get the category name and all fields by $category,
//$category->title;
//$catgory->name;
And if you are using twig in the page, and you are showing users with {{ user.name}}
(field may be different) then you have to just use {{ user.category.name}}
(field may be different) and you can show category too.
I hope you will get categories with users. If you find any difficulty please comment.