Twitter Communities is a new feature that allows users to create and join groups to engage in discussions about specific topics. The Subreddit/Facebook Group alternative segments Twitter feeds further by allowing users to send posts directly to a Community, rather than releasing it out for all to see on their feed.
Twitter Communities were created to give people a dedicated place to connect, share, and get closer to the discussions they care about most.
Communities are started and managed by people on Twitter — admins and moderators who enforce Community rules and keep conversations informative, relevant, and fun. People who accept invites to join a Community become members.

Tweets in Communities can be seen by anyone on Twitter, but only others within the Community itself can engage and participate in the discussion.
Everyone who is interested in joining a Community will need to be invited into the Community by a moderator or another member. Once a person becomes a member of a Community, they will receive five (5) invitations to invite more members.
Some Communities have open membership. This means that you can join without an invite. On Communities with open membership, you can tap or click on the Join button at the top of the Community’s page below the Community’s name. The Community’s rules will pop up for you to review. If you consent to the Community’s rule, tap or click on Agree and join.
A moderator (or mod) is someone who helps keep conversations safe and on track in a Community. They are chosen and overseen by a Community admin, and may be tasked with other duties as the admin sees fit.
An admin is the owner of a Community. Their duties may include keeping conversations safe and on track, reviewing member reports, hiding member Tweets, or removing disruptive members from the Community. They may also choose moderators to help with these tasks. Admins are responsible for managing the Community, which includes adding, modifying, and/or removing Community rules, name, and description. As the admin of a Community, you may invite as many moderators as you wish.