Check out Caedes and invi's lorea repository, they have plugins for subgroups and related groups which may help you. http://bitbucket.org/rhizomatik/lorea_production/src/c70a1719b47a/elgg/mod/

The Group Widgets plugin allows to create different group layouts as well. It's not sub-groups, but allows to define different group types.

http://community.elgg.org/mod/groups/topicposts.php?topic=577599&group_guid=52477 has some new notes re: SubGroups. I was working on collecting Groups by their initials to allow an alphabetical look-up and realized that a similar technique could be used to collect Groups as SubGroups of another Group ;-)

Did you find out or anybody respond as to how to do this.  Please let me know I am looking to do this as well

We are working on one now.  It is pretty specific to our situation so it may not be widely usable for anyone else.  We only need one level of sub groups (main groups and several sub groups attached to each main group) 

Our current model for getting this to work is to identify the "main" groups using metadata (similar to the featured groups).  From there we are simply going to add a relationship between the regular groups and its "main" group.

In the UI we will display the "main" groups first and the users can drill into the main groups to find related sub-groups.

The relationships are pretty basic but will serve our needs.  It is in the UI where most of our site specific features will take place.  Maybe we will release a basic version of this back as a plugin when we are finished.

WHat you mention is what I was thinking.  I was setting up a social n/w for Non Profits and wha twould be nice is to have a sub group within the group such as Donors, volunteers who have access to specific different resources.

When ar eyou expecting to release this.