Cant get the maddog site to work. Its hows page error.
Anyway, I have a solution her that works for small numbers of combinations.
Lets say you have x forums and wish subscribers to select subscriptions for qany combination of the x forums.
Each combination of forums is named its own name as a private group. eg someone subscribed to forum A, B and E is joining group ABE, Someone subscribed to forum A and C only is in private group AC
Manually you have to add group permissions to each forum. So in Forum A you must add private group AC and private group ABE.
Same for all other combinations. 5 differnt groups gives (maths lesson)
5C1 + 5C2 + 5C3 + 5C4 + 5C5 which equals 31 group labels.