We got a category for welcome pages, user guides, FAQs, etc. One of the topics outlines basic rules of behaviour, together with privacy and security concerns (ToS). We know that all invited users can be directed to land on that page, but we encourage users to send the link to others. As pointed out, there isn’t an explicit Discourse feature to force that behaviour on users who self-register (and Admins approve them).
We quite like the solution described by Stephen, as we were already considering carrot/stick approach: users got only level 1, and have to read and agree to ToS to be elevated to level 3. But I admit I find Stephen’s solution more entertaining… 