This study refers to them as casual smokers or social smokers:
We investigated how adolescents define different smoker types (nonsmoker, smoker, regular smoker, addicted smoker, heavy smoker, experimental smoker, casual smoker, and social smoker) using multiple indicators of smoking behaviors, including frequency, amount, place, and length of time cigarette smoking, and whether differences exist by smoking experience.
The percentages below indicate what the subjects thinks is a proper definition for casual and social smokers:
A casual smoker was characterized as an individual who smokes every few months (22.5%), a couple times a month (28.6%), or a couple times a week (27.8%). The amount of smoking for a casual smoker generally ranged from a few cigarettes per month to a few cigarettes per week (66.4%) and tends to smoke at parties (33.5%) or anywhere (36.2%). Over half of the sample (64.8%) characterized casual smokers as having smoked for a few months or for about a year.
Adolescents characterized a social smoker as an individual who smokes a couple times a month or week (62.5%), smokes a few cigarettes per month or week (57.1%), and tends to smoke at parties only (73.3%). Although the length of smoking for a social smoker was widely distributed, the majority of the sample (63.1%) characterized a social smoker as having smoked for a few months or for about a year.