There's a number of cases where ある can be used with animate subjects. The most common such case is when talking about a person having (a certain number of) family members, friends or guests - 私は兄弟が五人ある (I have five siblings). According to the book Essential Japanese Vocabulary, ある can be used to signify the existence of animate beings, especially a) if the subject is not a specific person or animal, b) if it's irrelevant where the subject exists, and c) if the noun signaling the subject is preceded by a relative clause. While b and c don't really apply in your example, it's true that specific participants aren't relevant for that sentence, and conference participants can be considered a type of guest, so I guess it's a permissible case.
BTW, since おる/おります is a humble word (謙譲語), you would only use it to talk about yourself or your in-group (people from your own family, company, organization, etc.) when addressing people from outside the group. I'm not sure how participation in the World Esperanto Congress works, but おります would only be used in official communications by the organizers about the participants if they are all formal members of the same organization. Otherwise, the neutral verb to use is いる/います.