Setting Community Followers Using the Default Members Group
Community owners can determine if the community has a default members group from the Admin tab or from the Members tab of the community. This group can be used to manage who follows the community.
Users added to the default members group is automatically made to follow to the community. Users may subsequently choose to unfollow the community, and that decision is respected even though the user remains in the default members group.
Users removed from the default members group is automatically unfollowed from the community, unless that user chose on their own to follow the community at any prior point in time.
If an audience is also being used to manage followers (see the Setting Community Followers Using SharePoint Audiences section), the user who has not chosen on their own whether to follow or unfollow remains a follower so long as they are in either one of the default members group or the audience. They must be removed from both to be unfollowed.
Regardless of any of the above, a user is always unfollowed from the community if it is found that they no longer have any site permissions to access it.