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. It only generates your home feed content based on the topics you are following. 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.
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.