You can access the user-management with rabbitmqctl and use the command:
add_user {username} {password}
or more preferably maybe edit an existing user, or set the permissions for the new user with:
set_permissions [-p vhostpath] {user} {conf} {write} {read}
For example use the following commands: (it is important to perform these three steps even when creating a new user, if you want to be able to login to the UI console and for your programs to work without facing any permission issues)
rabbitmqctl add_user newadmin s0m3p4ssw0rd
rabbitmqctl set_user_tags newadmin administrator
rabbitmqctl set_permissions -p / newadmin ".*" ".*" ".*"
…to create a new administrator user with full access to the default / vhost.
You can find all this on the RabbitMQ homepage, and more specifically on this page