You certainly would need a referral from the primary for a first-time visit to a specialist. However, follow-up visits to a specialist don't typically require another referral.
Additionally, there are different types of HMO plans. There's the large insurance companies that provide an "HMO" access. They may not own the entire organization like Kaiser Permanente or Humana, but their policy is that specialists must be referred by a primary.
My employer's primary plan is a PPO, but with an HMO like copay. I actually work for a pretty large company, and I'm pretty sure that we're self-insured. That basically means the company pays for all covered costs and puts money into a trust fund, but that a traditional health insurance company is contracted to get access to a provider network and to administer the program on behalf of my employer. Any "premiums" we pay for family members goes towards the trust fund. If a company is large enough, this usually works out better than paying premiums for large group plans. Of course sometimes it doesn't if the company isn't that large. I remember the CEO of AOL blurted out that their profits were hurt by a couple of covered dependents who had major health costs. He basically got roasted for it in the press because it was highly insensitive.
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. Most self-insured plans have a a reinsurer for large claims though too do it doesn't hurt them that much.
