Joomla EventGallery is one of many ad-ins available for magaing photo, but is successfully used by many Groups. To install it, first you have to download the "Event Gallery Core" for the developers web-site. At the time of writing, this was version 3.11.16 and can be found at www.svenbluege.de/joomla-event-gallery/eventgallery-download/event-gallery-core, but over time newer versions may well be made available, and the web address may change.

 First, download the zip file to your local computer (it will have a name like pkg_eventgallery_core_3_11_16_20201002154334_1cd32eef), then from your web-site backend, use menu Extensions->Manage->Install to upload the zip file from the location to which it was downloaded. When you have done that, menu "Components" will have a new entry for "Eventgallery".

There are many options available, and the component provides an "Overview" option wich explains how it should be used.