To add or remove a student from an existing snapshot, first go into their student record. Then go to the side-tab Snapshots. You will see there all the snapshots they are already a part of.:
Then, click Options > Add... which will bring up the list of snapshots you can add the student to. You can select multiple snapshots. Click OK.
To delete a student from a snapshot, go to their Snapshots side-tab, select the snapshot you want to delete them from, then click Options > Delete
It is the same idea for staff snapshots. All staff have a Snapshots side-tab. Follow the same instructions given above to add/delete.