How can I start a sub-group?

I'm currently a mod on ICMYTsupport@yahoogroups.co.uk for users of the PMckenna weight loss system. I would like to set up a sub-group to deal with "other issues" as we have for eg some people who need support for medical discussion but still want to continue with ICMYT. I would be the owner/moderator of the sub-group. Can I do it & if so, how? Thank you for your kind istance.