Try clicking to the "to ..." button just to the left of the "To Bar". You should see your category contacts there with two little pawns from a chess set just to the left of it. Click on it once and then click on the "to->" button just below and "+ abc", or whatever your category name is, will appear. Then just click ok.
Be aware that non-business email services tend to limit the size of categories to 20 or 25 generally. So you might have to make your categories fit what you email can take. It's a restriction of the service not the program.