Get behind a charity as a Life Group. Make it local and personal to what your group is about. For instance, The Wellspring is a homeless charity based in Stockport. They have a headquarters you can visit as a group on a regular basis to see what your money is being spent on. You may even want to get involved with more than just finance.
Giving to a charity as a group is great for several reasons: it gives you a common cause; it disciplines you in giving a certain amount every time you meet; it gives you common prayer points; it provides an easy route into mission.