When someone has suggested an idea for a new sub section, what happens next?
Does there have to be a minimum number of people expressing interest for it to be taken up?

It depends on the section or group, whether it's been considered or suggested before and whether there is sufficient interest in the subject.

If we're going to have a new group we also need some volunteers to help the group run smoothly.

The first step before starting up a new group is to gauge the interest amongst forum users for that subject with a thread asking if others are interested in the topic. After that there's a delay whilst the organising happens and we work out if there are any reasons why the group can't happen (there have been a number of groups which have not got off the ground for legal and copyright issues for instance) then the forum owner sets it all up and a new group is born.

We do occasionally set up new forum areas, rather than groups, if there's sufficient interest in a specific area.