Echo had heard about the bbq and decided to see what it was all about. While she knew Gibbs she didn't spend a lot of time with the man. When she arrived she didn't see that many people, but she did show up early so she didn't expect to see that many people.
Finding a drink and then a seat off to the side she simply watched everyone else while she drank her soda. While Echo had come out of her shell while she had been on the island, she still often remained to herself it was easier that way for her.