Slow and fast metabolisms do actually exist. We all have differing levels of thyroid hormones and, in short, the higher your free T3 levels the faster your metabolism and the higher your TDEE. Hence why T3 is a popular drug among bodybuilders during cutting phases. However:
While we all have differing metabolisms, EVERYONE has a Total Daily Energy Expenditure. Could be 2000 calories per day, could be 5000. The simple fact is, you need to eat more calories than you burn as your TDEE to gain weight, and since we all have a TDEE, we can all gain weight, even if it means eating upwards of 5000 calories a day.
Problem with people that swear they eat loads and don't gain weight, though, is that they really don't eat much. If you calculated the caloric intake of all the food you eat on the average day, the figure probably wouldn't be huge. It's like on Supersize vs Superskinny where the skinny person swears they eat loads and the fat person doesn't know why they're fat; then the experts monitor their eating habits and it becomes obvious. Then they try each other's diets and the skinny person struggles to eat it all and the fat person is constantly hungry.
For the record, I've been the "I never put on weight and I eat loads!" guy, but looking back I now know that I used to eat **** all really