This is the 6032nd most frequent Russian word.
совместное
Joint; shared; collaborative.
The word 'совместное' here describes the nature of the event as being shared or collaborative.
Мы организовали совместное мероприятие для учеников разных школ.
We organized a joint event for students from different schools.
In this sentence, 'совместное' characterizes the shared effort or activity of the participants, indicating collaboration.
Совместное участие в проекте помогло участникам наладить новые контакты.
Joint participation in the project helped the participants establish new connections.
Here, 'совместное' emphasizes that efforts are unified and collectively contributed towards a common goal.