Unless MAAS was installed from the Ubuntu Server ISO, you'll need to create a user in order to access the web UI. This initial user is a MAAS administrator:
sudo maas createadmin --username=$PROFILE --email=$EMAIL_ADDRESS
sudo maas createadmin --username=admin --email@example.com
The username can be anything. You will also be prompted to supply a password for the user. There is a command option ('--password=$PASSWORD') that can be used to specify one but depending on your environment this may pose a security risk.
Note: At this time MAAS does not make use of the email address. However, it may do so in the future.
You can now log in here: http://<region controller address>:5240/MAAS
Once logged in to the UI an administrator can use it to create regular users as well as additional administrators. See User Accounts for this.
See MAAS CLI for logging in via the CLI and working with the CLI.