1. to add; to add up; to sum up; to append; to annex
そこに油を少しずつ加えていく。Gradually add oil to it.
2. to increase; to gather (e.g. speed); to pick up
雨はますます激しさを加えた。The rain grew heavier and heavier.
3. to include; to count in; to let join
悪いが二人についてはメンバーに加えることは出来ん。Sorry, but I can't add the two of you as members.
4. to inflict (damage); to deal; to give
振り上げた拳で、真上から一撃を加えるつもりだった。I meant to deal a blow from directly above with my raised fist.