Yes, you can call a function from within a loop (but do not define the function in the loop - you need to define a function only once). You do this by putting the function call block inside the loop just like any other command you’d use in a loop.
No, students cannot collaborate simultaneously in App Lab a la Google Docs.
When students need help with their code, generally we encourage them to get help from classmates and the teacher. There is no official online community designated for this purpose of outside help that I know of. Sometimes teachers will post “Share” links on this forum to get help from other teachers on behalf of their student.