For that you'll need to have a 'Volunteers' table where each record represents a single volunteer, and then you'd rollup on the 'Hours' field with 'SUM(values)':


Link to base
From your screenshot, it looks like the 'FFN1' field is the one that contains the volunteer information? If so, you'll need to create a linked field to the 'Volunters' table in this table, and to populate the data quickly you could:
1. Create a new view with no groupings, with the FFN1 field and the linked field to the Volunteers table visible
2. Click the field header of the FFN1 field, which selects the entire column
3. Hit CMD/CTRL + C
4. Click the field header of the linked field to the Volunteers table
5. Hit CMD/CTRL + V
This will link up all the records, and when you head to the Volunteers table you'll see one record per Volunteer has been created for you
If you've already got a 'Volunteers' table, then instead of using the 'FFN1' field, use a lookup field to the primary field of the 'Volunteers' table instead!