Less simple

membership.rb

class Membership

  belongs_to :user
  belogns_to :group

  after_save :send_notifications, if: ->(m) { m.accepted_changed? && m.accepted? }

  def send_notifications
    NotificationMailer.accepted(self, user).deliver!
    PushNotifcations::NewMembership.new(self, user).dispatch!
    
    group.admins.each do |admin| 
      NotificationMailer.accepted_admin(self, admin).deliver!
      PushNotifcations::NewMembershipForAdmin.new(self, user).dispatch!
    end
  end

end