We are a group of 5 stundents but we don't know if everyone should join or only me.

Btw is there any problem of using unity? We have a subject with which we will use unity and we want to take the opportunity to learn about video game development and also learn for the subject

The way to join as a team is to either make an account for the entire team or have one person submit the games and then you can add them as admins as seen here:

And for the second I'm not one of the hosts this but it seems like the only rule is to do an entirely new engine so I think its fine