There's loads of students at UC Berkeley who are passionate and active around environmental issues, ranging from organic gardening, to reuse programs, to converting the campus to clean energy! Unfortunately, the plethora of groups available can sometimes make it hard to find the one you are looking for. Fortunately, the Berkeley Institute of the Environment has put together a fairly comprehensive list of the various student groups active on campus.
The list can be found at http://bie.berkeley.edu/studentorgs
A few of the group's websites are a little out of date, but make sure you contact them before assuming the group has gone the way of the dodos - sometimes student groups are just a little slack about updating their sites.
The BIE site also has some other great resources, including a list of Berkeley classes addressing environmental issues, so make sure you explore the site.
Another great way to find out more about the different environmental groups on campus is to attend and support the newly-formed Environmental Forum, a coalition body design to facilitate communication between different environmental groups on campus. The Forum typically meets once a month. Email berkeleyenvironmentalforum@gmail.com to find out when the nest meeting is being held!