party.rb

class Party < AR::Base
  belongs_to :user
  belongs_to :session
end

session.rb

class Session < AR::Base
  has_many :parties
  has_many :users, through: :parties
end

user.rb

class User < AR::Base
  has_many :parties
  has_many :sessions, through: :parties
  has_many :fellow_party_animals2, -> { where.not(id: self.id).uniq }, through: :sessions, class_name: "User"
  
  def fellow_party_animals
    User.joins(:sessions).where(sessions: { id: sessions }).where.not(id: id).uniq
  end
end