The Brilliant community is a great place to work on problems created by other folks who love learning with Brilliant, engage in discussions about math, physics, and computer science, or pose your own questions to others!
You can sort community problems and discussion questions by topic, popularity, and difficulty, and when you're ready to pose your own question, just click the "start contributing" button at the top of the page. You can also use this same button to start creating wiki pages, contributing to the ever-growing brilliant wikis.
Additionally, if you're interested in diving into a new subject, you could always read up on the Wiki of the Day, or revisit an old problem set with the Problems of the Week Archives.
Note: If you're looking for the community page on the mobile app, it's in the sidebar, rather then the main menu bar, as shown below.