By default, the front-end tag selector will show tags used by events in the current group/view.

You can override this to show specific tags on a per-group basis by adding this to your public.config.php file.

$_LW->CONFIG['CALENDAR_GROUP_TAGS']=[ // custom tags to display on group calendar(s)
        // Group ID => Array of tags
        1 => ['Tag 1', 'Tag 2'] 
];