$ mongo
> db = db.getSiblingDB('admin')
> db.createUser({user: "username", pwd: "password", roles: ["userAdminAnyDatabase"]})

$ mongo
> db = db.getSiblingDB('admin')
> db.createUser({user: "username", pwd: "password", roles: ["dbAdminAnyDatabase", "userAdminAnyDatabase", "readWriteAnyDatabase", "clusterAdmin"]})

References:

• http://www.mongodb.org/display/DOCS/Security+and+Authentication
• http://docs.mongodb.org/manual/tutorial/add-user-administrator/