user.rb

team.rb

class Team

  include Mongoid::Document

has_and_belongs_to_many :captains, class_name: 'User', inverse_of:  :team_captained, autosave: true

has_and_belongs_to_many :members,  class_name: 'User', inverse_of:  :team_joined, autosave: true
  
 references_one :owner, class_name: "User", autosave: true 

end

user.rb

class User
  include Mongoid::Document  

  has_and_belongs_to_many :team_captained, class_name: "Team", inverse_of: :captains 
  has_and_belongs_to_many :team_joined, class_name: "Team", inverse_of: :members
  referenced_in   :team, inverse_of: :owner

  ## rest of user code

end