Adding another user as a member of a space makes them a peer of the space, and grants him the same privileges, as well as being able to create, edit, delete and publish content, as a member they are able to add and remove members from the space.
This is analogous to cutting a key and giving it to a house guest. He may turn change the locks and lock you out, but that's unlikely to happen with people you invite into your home.
This is analogous to cutting a key and giving it to a house guest. He may turn change the locks and lock you out, but that's unlikely to happen with people you invite into your home.