You can create a new volume in the Organization application:
To correctly fill in the required fields, you can use the existing volume as a basis.
You would typically create new volume if volume contains so many files/directories that volume operations become slow or you start coming close to the OS limit on inodes.
Keep in mind that, if you create new volume and move users over to new volume, you will see that # of files in the old volume will start to go down over period of time. This is becuase, as users release datasets, purging action will start to delete older files from volume and you will see that older volume start to shrink again.