I would get involved in activities myself, try to ask them questions, invite them to group activities, praise them. However, it is important to understand that they do not have to participate in all activities if they do not wish so.
I was in the summer schools myself, and I think that Activity leader must be responsible for the students and make sure that they are in the safe environment. Moreover, AL should provide a very friendly environment so students from different countries would get to know each other.
message / call the student, if they are not picking up, finding out who they were with and ask them where the student is. If they are not back and the bus need to leave, leave one of the leaders behind to wait for them/search them.