Users
Vulcan uses the standard Meteor user accounts system, along with the std:accounts-ui package for the front-end. 
Preloaded Properties​
A number of properties (such as a user's _id, email, etc.) are preloaded for the current user when the app loads. These properties are controlled by the UsersCurrent fragment:
fragment UsersCurrent on User {
  _id
  username
  createdAt
  isAdmin
  displayName
  email
  emailHash
  slug
  groups
}
You can extend or replace it just like any other fragment.