Are you an admin of a group page in the Community?
Unsure of what your powers are vs. everyone else's?
As a group administrator, you are able to customize the group Overview page and manage members (which means disabling them, deleting them, or sending them messages).
You'll see a "Manage" button in the upper right corner of the page. This will allow you to do all of the things describer here: Managing a Group in the Tableau Community
Non-admins will be able to create whatever content type that you allow in your group, as well as to respond, "like", and bookmark any content. Additionally, they can invite other people to the community - but ultimately, as the group admin, you have the ability to accept or reject someone's membership.