The Students Union is one of the largest centres for volunteering in any University!
You can volunteer for committee positions and roles with clubs and societies or in a peer support group. Just being a member, whilst its a great thing to do whilst you study, isn't volunteering but being a President, Vice President, Treasurer or Officer will give you valuable transferable experience and should go straight on your CV.
Alternatively, you can volunteer directly with the Union itself. If you would like to help deliver student services, advice, activities and events see their in house volunteering positions. Or why not consider running for office as an elected student representative?