Groups act as small communities under your main community. In groups, you can make limitations and restrict the access to the content. While everyone in the community can freely follow a topic and topics are used to organize the contents. Tags (topics) are assigned to a content to make it easier for members to find the content relates to a specific subject.
Following a topic does not connect people with same interest together. While in the groups, you are connected to the other people who have the same interests as yours.
Groups have the ownership, and you can have your team members or even your customer to own their own groups. Topics does not have ownership.
Also, groups makes it possible to customize the home feed for each user based on the group(s) that a user is a member of. You can manually add someone into a group, or you can define "default groups23" where all your community members will be a part of automatically.
You can also use our API26 to automatically connect a group of people on your main website (for example people on different subscription on your website) into specific group(s) inside your community.
We are working on the process of tagging contents with topics or choosing a group to post the topic in. Our goal is to design a behavior in which a community member should either post in a group or post under a topic. We will share more details when we get closer to the date of releasing this feature.