Communities are how DNR decides who you are competing against. A community is one of your Discord servers, so you race the people you already race with rather than a global board full of strangers.

Open Daniel Newman Racing > Communities. The page lists the Discord servers you are in and lets you choose which ones you want to take part in, with All on and All off shortcuts, and a member count against each. Set faster times, climb the board, and beat the people you race with.

This matters because Track Rivals leaderboards are per community. If a board is empty, or you see "You're not in a community yet", it is because you have not joined one here yet.

You need your Discord account connected to your DNR account for your servers to appear. If the page says no Discord servers found, connect Discord in Dashboard > Profile > Connected Accounts on the website.