Software developers. There are loads of new features in Lion (new UI, versions, auto save, all that stuff) and general changes/bugfixes that could break apps. You want everything to work on day one, and to take advantage of the new features in Lion, right? That's why developers get early access to OS releases, so they can prepare their apps to work on day one (or even before then!)
Apple's developer system has two tiers: free and paid ($99). Developers with free accounts get access to all the dev tools and documentation, but can't submit apps to the AppStore (on OSX, they can still make and distribute apps themselves, on iOS the app is limited to an iPhone/iPad simulator on a computer).
Paid members are allowed to submit apps to the store, get tech support from apple, and get early access to OS releases
You can join the developer's program for free but you only have access to the SDK and compiler. The 99 fee to join provides benefits, like access to betas and support should you run into issues developing a program.