As popular as it is for people to want to lose weight, some people (admittedly mainly guys) want to increase their weight i.e. add muscle - as muscle weighs more than fat.

Most consultations we have when speaking to people who want to add weight/size usually result in us increasing calories and dropping expenditure. Put simply, people don't eat enough. In order for your body to build muscle and increase weight you need to be in a calorie surplus.
 This means your body needs to take in more energy (food) and expend less energy. Unfortunately it is next to impossible to build muscle and add size while being in a calorie deficit. Too many people want to stay lean, do their cardio and be in a calorie deficit.

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If you're looking to put on some weight/muscle you have to understand it is a very slow process, losing weight is arguably considerable easier, to an extent. Youll have to be very patient and careful as ideally you want to minimise the amount of "body fat" you put on to increase weight and rather increase the amount of muscle you wish to put on. 
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If your struggling to put weight on, think about your energy expenditure and overall calorie intake, carbohydrates and protein are key in building muscle.