Where 7.3 does have a more granular permission system, there is no permission to allow users to only reply to their own posts...

There are several you could play with, but I doubt you'll be able to achieve this via a stock install:
Can reply to existing topics
Can read topics
Can create new topics

So you could make it so that they can create but not read anything, but not so that they can create and only read that which they've created...

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