To find fellow writers and join a Writing Group, check out the "Writing Groups Assemble" forums. There, you can request to join a Writing Group and secure an invitation from a Writing Group Administrator.
To create your own Writing Group, go to the "My NaNoWriMo" tab and click "Groups". Then, click the button labeled "Create a Writing Group". You can then recruit members through the forum linked above, or by typing in usernames on your new group page.
There is no limit to how many Writing Groups you can join, so your options for building community are limitless!