You can post in these forums like you are right now. You just are not supposed to offer and services until you hit the 100 post level. It really depends on the situation too... Im sure some mods would let it slide if you were at 50+ posts... it's basically a rule to keep hosts from just joining for the purpose of getting clients.

Isn't it possible to set up group permissions in such a way that people were physically unable to post or reply in the OFFERS sections, or reply in the REQUEST sections, until they had the minimum number of posts?

Yes, there are people who read the rules (dacsoft - gold star! ), but having such a mechanism in place would keep things from deteriorating.