I have been doing some testing.
My site map http://tradr.com/sitemap.xml is only listing 21 groups.
Went to site map and searched for "tradr.com/groups/"
I have 550+ groups.
It is supporting BP Group Hierarchy, as one of the groups is a 3 tiered group.
Therefore, it seems site map is only displaying the most active groups, and not all of them.
I have clicked, update my site map, recently too.