See the 'Debate challenges' thread in the parent board of this one.
Regarding inviting additional participants into a debate, it would be up to the primary participants - though I think it would somewhat unbalance the debate if each 'team' had different numbers of members, so I would not generally support the ad hoc inclusion of guests into a discussion. That, to my mind, is what the regular discussion threads are for, not this section. Non-participants are free to add their comments on the ongoing debate into the 'commentary' thread, of course, and nothing is stopping participants from using any material posted in the commentary thread in their own arguments (there's no realistic way one could police that).