Problem is subdomain.domain.com and domain.com are looked as 2 different domains in eyes of trade script. So bets would be to do domain.com/niche, so directory instead of subdomain.
Then you can make groups for each niche, assign trades to each group and send from that niche page to group. You can also ask traders to send to niche landing page so domain.com/niche. That way is optimum for getting niche traders live.
Keep in mind this method needs some tweaks in your CMS and maybe even line or two of custom coding so best bet is to get someone advanced to setup it for you.
