How can I help my cat gain weight? She has been checked by the vet and is healthy, she's just skinny.?

I adopted a cat from the animal shelter (where I volunteer) to give sammy (my other cat) a friend and because she was an olderish pure black that we were having trouble getting adopted. She is so loving and an amazing girl. She has never lived in a home, she is a pure stray, but doesn't act it. However, she does look it. She has been seen by vets and is in good health, but she is so dang skinny. She is only 2 months younger than Sammy at 7 months and less than half his size. You can feel her ribs and see her bones when she sits. She eats quite a bit (that I see) but I don't know if I am giving her the right food or if I need to add something into her diet. I want to get her at a healthier weight, and I know it will take time, but I want to know if there is anything else I can do to help her along.