Final answer:
The serving style at the poolside party was a buffet, where guests moved in a line by the food table to be served.
Step-by-step explanation:
The poolside party of Charlie's friend used the buffet serving style. This is when food is laid out on a table and guests serve themselves or are served by attendants while moving in a line along the table.
This method is efficient for serving large numbers of people and allows for a variety of dishes to be presented and chosen according to individual preferences.