Contributor status is what you (generally) get when you're invited to danbooru. This status allows you to browse the full contents of the site and avoid ads. You also don't have to have your uploads approved anymore.
This is my experience, on a scale from hard to easy (and from most to least likely to garner an invite to contributor status):
- Develop something for danbooru
- Translate
- Contribute to the forum with ideas, get involved, help out
- Upload good material, tag it properly
- Have a friend among the mods that wants to invite you
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Donation has also a recently been added as an option which will allow you to pay for a privileged account. The privileged account is the same as the contributor account with one exception: your images still need to be approved by a mod.
For more info: http://danbooru.donmai.us/help/users
Updated