I gave AFF and FT a sabbatical yesterday and in doing so probably missed the thread referred to. khe serah serah ... (ap. Doris Day)
My perception in regard to AFF is that having a 'non public forum' may not be in the in line of what admin is trying to achieve.
Technically it can be done, and posted there are several way to do so.
For example, many of the more prolific posters on this forum would know of a couple of FT forum that have access restrictions. One is "Coupon Connection", another "OMNI". Both require a minimum number of posts and a period of membership before having access. Outside of FT, there is also the paid subscription and referral criteria for other fora.
As already posted, PMs are perhaps the best way to deal with such issues; one can send to more than one forum member at a time and thus 'private' issues can be adressed.