groups and "meta-groups"


i need to establish a community in which every user is member of a group consisting of 10 people, a so called core group, from the beginning. in addition to that there should be  "meta-groups" gathering the members of 10 core groups.

then every user should be able to reach his core group and his "meta-group" directly by hitting a link on his topbar...

any ideas how i could do something like that with least efford??

hoping for suggestions!