Members Can Add New Members Through SharePoint
There is a permission security hole. Members can add other members to a Team by using the SharePoint online interface and editing the group membership. Only Owners should be able to add new members. There is also no log of this in the general channel
For public teams, this is a feature. That makes it easier for IT admins to create teams.
However, for private teams, this is not possible. Can you please see if the team you’re looking at is private or public?