Assuming you're talking about Steam groups for the gaming platform Steam, there are a few ways to go about finding them.
The most obvious way is to simply log in to your Steam account and look at the left-hand sidebar. Under Community, there is a section called Groups. Clicking on that will show you all of the groups to which you currently belong. You can also browse through groups that you don't belong to, and request to join ones that look interesting to you.
If you're looking for more specific kinds of groups, or want to find groups outside of the Steam interface, there are a few other options. One is using dedicated group-finding websites like steamgrouplistings.com or steamsocial.net. These websites allow users to search for groups based on game, activity, location, language, and other criteria.
Another option is simply doing a web search for something like Steam Groups + [game name]. This will bring up results for any number of different kinds of groups, including fan communities, clan/team recruitment pages, trading/gifting circles, and more.