As codinghorror posted, access to Categories (and Messages) can be easily controlled.
What I’m wondering about is what you mean by “early access”.
If you mean “before everyone else”, then not so easy.
But I can think of a couple of ways.
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Start topics in a restricted category (or as Messages), then move them into an “open” category later.
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Unlist topics, send the links to the topics to the group, then list the topics later.