Drinking alcohol in and of itself will not affect muscle growth. Some people may notice that when they are in the habit of drinking a lot they are losing muscle mass or are generally more out of shape. This could be for a variety of reasons. One is that drinking alcohol may lead to hangovers and a loss of interest in working out.
After a night of drinking, you may not feel as motivated to exercise or work out really hard. Alcoholic drinks also have a lot of calories which could make someone gain weight and feel more out of shape.