Your username is globally unique, but will not float above your avatar or be given to others near you unless you choose. Usernames may be required for entry into some servers.
Your display name is not unique. It can be set in Preferences once you are in world. By default, there is no display name over your head.
All users have access to our forum (we'll let you know more about that shortly) where you can engage the rest of the group with discussions, questions, and share what you're building. You can login with the credentials you have chosen on the left.