I would also love to have groups in groups, or even the categories option discussed above! I have been working on mapping different neighborhoods in my city (over 50!) with each neighborhood having points of interest added in. Due to the sheer number of neighborhoods, they have been grouped by region (North, South, Downtown, etc.) and so when a point is added for a specific neighborhood, it falls under the regional group that neighborhood is in. This has made locating, editing, or getting information about these points difficult, especially for those who are not familiar with the area/map.
I absolutely love MapHub, and it has become a tool that is used almost daily to convey localized info. I feel that adding this in, even if not used by most, would make this system truly the best of its kind.
Thanks for all you do, this is truly an amazing resource!