Please use the Question Tags to state the name and full version number of the Story Format you are using, as answers can vary based on this information. I will assume you are using the latest version of Harlowe which is v3.0.2
While you can use the (passage:) macro to determine which Passage Tags have been assigned to a specific Passage Name (defaults to the current one) there is however no built-in means to programmatically get a list of Passage Names within your story HTML file, which makes it difficult to loop through those passages looking for a set with a specific Passage Tag.
note: Harlowe has been deliberately designed to restrict an Author's access to it's JavaScript based engine, the story format has limited JavaScript support and doesn't have a documented JavaScript API.
If you were willing to use JavaScript to hack into the story format's engine to access its undocumented features then hypothetically you would need do something like the following:
a. Use the Harlowe 3.x Passages.getTagged(tagName) function to access obtain an Array of 'Passage' Map objects, where each such Map object repesents a Passage that had been assigned the specified Passage Tag.
var passages = Passages.getTagged('name-of-passage-tag');
b. Loop through each of the found passage Map objects and use the Map.get() function to obtain the Name of each of those passages.
var names = {};
if (passages.length > 0) {
passages.forEach(function (passage) { names.push(passage.get('name'))});
}