not simple at all

membership.rb

class Membership

  belongs_to :user
  belogns_to :group

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

  def send_notifications
    Resque.enqeue(NotificationMailerWorker, self.id, user.id)
    Resque.enqeue(PushNotifcations::NewMembershipWorker, self.id, user.id)

    admins = group.admins - user
    admins.each do |admin| 
      Resque.enqeue(NotificationMailerForAdminWorker, self.id, user.id)
      Resque.enqeue(PushNotifcations::NewMembershipWorkerForAdmin, self.id, user.id)
    end
  end

end 

class NotificationMailerWorker

  def perform(membership_id, user_id)
    membership = Membership.find(membership_id)
    user = User.find(user_id)
    NotificationMailer.accepted(membership, user).deliver!
  end

end

class PushNotifcations::NewMembershipWorker

  def perform(membership_id, user_id)
    membership = Membership.find(membership_id)
    user = User.find(user_id)
    PushNotifcations::NewMembership.new(membership, user).dispatch!
  end

end

class NotificationMailerForAdminWorker

  def perform(membership_id, admin_id)
    membership = Membership.find(membership_id)
    admin = User.find(admin_id)
    NotificationMailer.accepted_admin(membership, admin).deliver!
  end

end

class PushNotifcations::NewMembershipWorkerForAdmin

  def perform(membership_id, admin_id)
    membership = Membership.find(membership_id)
    admin = User.find(admin_id)
    PushNotifcations::NewMembershipForAdmin.new(membership, admin).dispatch!
  end

end