These are the indexes Meteor currently ensures on the users
collection -- from accounts_server.js
/// DEFAULT INDEXES ON USERS
Meteor.users._ensureIndex('username', {unique: 1, sparse: 1});
Meteor.users._ensureIndex('emails.address', {unique: 1, sparse: 1});
Meteor.users._ensureIndex('services.resume.loginTokens.token',
{unique: 1, sparse: 1});