Github is used for a variety of activities in this course, including project management, team collaboration management, source code management, version control, making documentations using Github's markdown, sharing feedback on team project work iterations, etc.
Piazza is the communication channel. If you have any questions or concerns, make a post on Piazza. You can do either public or private posts (annonymously if you want!). If others could potentially benefit from your question, consider making it public. Never send your homework solution code in a public post.