Issuing sub assets to Asset holders?

Hi guys.

Searched the site but have been unable to find the information I need.

Basically if I have an asset I have created which people now own. How do I issue sub assets to those holders?

Additionally, is there somewhere people could then trade those sub assets?

ie. I have an asset issued to each member of an organisation I run. Now Each member will receive a sub asset for their contirbutions to the organisation. Where could they then trade those sub assets?

Thanks in advance.

Oinkster