Bartending should be one of the features offered by your events or equipment company. They must be able to provide you with one or two who can man the bar and serve specialty drinks to visitors. And this is perhaps the biggest reason why you should get a barista: he will help you take care of your guests. If they need anything related to drinks, your guests wouldn't have to bother you. They will go straight to the bar and the will ask the bartender. This means you have one less task to worry about.