Use the tools on this page to find online and offline groups of all different types. Interest groups, clubs, associations, etc.
These sites are various forms of online message boards, also called forums. Some of them are also mailing lists, meaning members can receive and send messages through email.
LinkedIn Groups requires a (free) account in order to view search results. Even those that can be read by anyone require signing up in order to contribute, and some can only be joined with moderator approval. LinkedIn groups are often business-oriented, while the rest are general-purpose.
Reddit is a bit different than the others, in that in addition to posting text and links and commenting on those, users can also vote for or against each post. This means that the home page of each Subreddit (as they are called) shows what is popular at the moment.
This tool uses Google to find all sorts of groups by adding group-related words to your search.
Meetup is used mostly so that people with common interests can find each other online, and meet in person.
GroupSpaces helps existing groups organize themselves online. They do not have their own search, so this uses Google.
See also Nextdoor which host online discussion for neighbourhoods in the United States and some other countries.
The Yearbook of International Organizations, by the Union of International Associations lists a huge number of non-governmental organizations and intergovernmental organizations, although the details of each one are limited for non-paying members.
CareerOneStop lists professional associations, mostly in the United States. The Trade Association Forum lists those in the United Kingdom, and the Government of Canada lists business and trade associations there.
Wikipedia is far from the only wiki. Fandom hosts wikis on all sorts of topics.
DailyStrength and Online Support Groups (which has no search) both host online support groups for a variety of situations. Psychology Today helps find in-person support groups in the United States and Canada. You can search by location or group name.
Lists on Twitter are collections of Twitter accounts that have something in common. Often those listed form a loose online community. Find a Twitter List is a third-party tool for finding these lists.
Quora is a question-and-answer site, and people who frequently ask and answer questions on the same topic can form a loose community.
These are individual websites for specific interests that contain groups within them. Flickr Groups revolve around photographs, DeviantArt Groups connect artists and their fans, and Goodreads is for book-lovers.