This will be a tricky one, but it's really worth it. So, basically, you need to edit the name of your "Registered Users" usergroup, and set the name "Probation".
Create new usergroup, and name it "Registered Users", put the same permissions as your last Registered Users usergroup had. Move all users from the old Registered Users (it's called "Probation" now) to the new one, so the old users wouldn't be affected by the changes.
Enable the captcha for the "Probation" usergroup, go to Usergroups - Promotions - Add new promotion:
Usergroup: Probation
Reputation level: leave blank
Days registered: leave blank
Posts: 5
Promotion strategy: posts
Promotion type: primary usergroup
Reputation comparison type: nothing
Move user to usergroup: Registered users
That's all. Users would be forced to enter captcha, and they would be in the Probation usergroup til they reach 5 posts.
