We know that guest has that option when posting and when you join the site, But what about an plugin for certain boards and able to pick what group that has it enable.

Example, I have an group named "semi New"  after they made 5 posts they become full member, And each of there first post needs to have an reCAPTCHA.

 

PS: I have no clue if there is an plugin or not, but no settings

Also to add to the reCAPTCHA for all groups or permissions is to add reCAPTCHA when you download an files