The bodybuilding community still believes in the Positive Caloric Balance hypothesis i.e. a calorie is a calorie is a calorie. Their advice to gain muscle is: Eat more. This forum is dedicated to low carb diets and as such we're a little more informed on the subject so we would be the first to consider that advice as naive if not completely wrong.

Carbohydrates drive insulin drives fat accumulation. This fundamental principle is the simplest explanation of how we grow fat. Bodybuilders don't escape this mechanism in the least. In fact, the classical method to grow muscle is to eat more regardless of macronutrients, grow fat because of the high carb content, then cut by cutting total calories (which invariably cuts total carbs) again regardless of macronutrients all the while lifting heavy weights. In other words, even as they lift the bar i.e. exercise, they grow fatter. It's called the bulk/cut method. Do you want to grow fat? If not, then don't follow this advice.

The advice to eat more carbs comes from the fact that protein requires insulin to be used by cells. Since carbs also require insulin and more insulin would logically improve the amino acids uptake by cells, eating more carbs would then be the logical thing to do to gain more muscle. The problem with this logic is that it doesn't take into consideration insulin resistance. This builds over time such that cells simply refuse to take in any more insulin. At that point, the only thing that grows is fat tissue because it's the last tissue to become insulin resistant. It can take years like it can take weeks.

Carbohydrates serve only one purpose and that's fuel. It can't be used for repair or maintenance or any other purpose than fuel. Fat can be used for fuel, repair, maintenance and building blocks of cells, sterols production such as cholesterol and subsequently testosterone, and various tissue with the main tissue being the brain. Protein can be used for fuel, repair, maintenance and building blocks of cells and various tissue with the main tissue being muscles. Vitamins and minerals are used in every step of fuel utilization, repair, maintenance and building blocks and there are many more better alternatives than eating carbs to get those.

So yes, it's entirely possible, perhaps even easier, to grow muscle while in ketosis.