A person or people who accompany somebody who is too cheap to buy his own drinks at a bar and instead brings a flask of alcohol and orders soft drinks. The "flask beards" accompany the flask-bearer, ordering enough alcoholic drinks to disguise the fact that the flask-bearer is totally dumping outside liquor in his diet Coke. Typically the flask-bearer compensates his flask beards at a rate of about 25%, which provides incentive for the flask beards.
Kevin wanted to bring a flask of vodka to the bars on Saturday so he could save money by ordering only orange juice all night. To avoid detection, he recruited Pete and Bob to be his flask beards, so he would avoid detection.