@xrevan86 I suspect that non-local groups aren't shown via API simply because you cannot do anything with via API with non-local groups: join, leave, list members...
In a case of GNU Social seeing local groups only is mostly useless: main power comes with using the same group across different instances. E.g. naturally I would like to join !gnusocial But as I cannot even find this group via API from loadaverage.org, it looks like implementation of "list local groups" won't worth its efforts :-(