class User
  include Mongoid::Document
   has_many :offers, class_name: 'Offer', inverse_of: :buyer
   has_many :cobuyer_offers, class_name: 'Offer', inverse_of: :cobuyer
end

class Offer
  include Mongoid::Document
    belongs_to :buyer,  class_name: "User", inverse_of: :offers
    belongs_to :cobuyer, class_name: "User", inverse_of: :cobuyer_offers
    belongs_to :modifier, class_name: "User", inverse_of: nil
end

Offer.create!(buyer:User.new,cobuyer:User.new,modifier:User.new)

 Ambiguous associations #<Mongoid::Association::Referenced::HasMany:0x00007fe4e13faac0 @owner_class=User, @name=:offers, @options={:class_name=>"Offer", :inverse_of=>:buyer}, @extension=nil, @module_path="", @polymorphic=false, @validate=true, @class_name="Offer", @primary_key="_id", @klass=Offer, @default_foreign_key_field="buyer_id", @foreign_key="buyer_id">, #<Mongoid::Association::Referenced::HasMany:0x00007fe4e1407748 @owner_class=User, @name=:cobuyer_offers, @options={:class_name=>"Offer", :inverse_of=>:cobuyer}, @extension=nil, @module_path="", @polymorphic=false, @validate=true, @class_name="Offer", @primary_key="_id", @klass=Offer, @default_foreign_key_field="cobuyer_id", @foreign_key="cobuyer_id"> defined on User.
summary:
  When Mongoid attempts to set an inverse document of an association in memory, it needs to know which association it belongs to. When setting :modifier, Mongoid looked on the class Offer for a matching association, but multiples were found that could potentially match: #<Mongoid::Association::Referenced::HasMany:0x00007fe4e13faac0 @owner_class=User, @name=:offers, @options={:class_name=>"Offer", :inverse_of=>:buyer}, @extension=nil, @module_path="", @polymorphic=false, @validate=true, @class_name="Offer", @primary_key="_id", @klass=Offer, @default_foreign_key_field="buyer_id", @foreign_key="buyer_id">, #<Mongoid::Association::Referenced::HasMany:0x00007fe4e1407748 @owner_class=User, @name=:cobuyer_offers, @options={:class_name=>"Offer", :inverse_of=>:cobuyer}, @extension=nil, @module_path="", @polymorphic=false, @validate=true, @class_name="Offer", @primary_key="_id", @klass=Offer, @default_foreign_key_field="cobuyer_id", @foreign_key="cobuyer_id">.
resolution:
  On the :modifier association on Offer you must add an :inverse_of option to specify the exact association on User that is the opposite of :modifier.