Don't know about 500 users, but with around 100 I used just the simple blog-format (bl0gspot, started years ago). First got gmail addresses from expected participants, which was 'kinda complicated', then made a 'collective gmail' with shared password & email (and the ones who wanted to were invited as authors, and access limited to 'authors only'). People post and comment willingly. With tags and links it's easy to manage - for someone not ready to get into software intricacies
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