concert (a musical entertainment in which several voices or instruments take part)

celebration

"offline meeting": a real-life meetup between people who met online; IRL meeting

event where fans can take images with idols

meetup where people get together to casually study, work, read, etc. on their own

research association; research society / group of enthusiasts (as may be found in schools, colleges, etc)

the Orthodox Church

symposium, forum

ball (formal dance event)

consolation party