Often times when I talk to people who already donate to charity, they chose the charity due to a strong personal reason and I feel it is not my place to persuade them otherwise. For example, a veteran that donates to a NPO helping other veterans handle PTSD; a Type 1 diabetic that donates to a Type 1 diabetes research NPO; an adult who was an orphan that donates to local children's hospitals.
On the other spectrum, I know people who just aren't interested in charity at all - which I argue is most people in the US.
Perhaps the easiest would be people who are donating to charity, but don't have a strong personal tie to the charity?