Hi,
The Community Group Forums are not indexed for Smart Search, there's a standard SQL search used - mostly by
Group forum search results webpart.
To make the group forum to be indexed by Smart search, you could change the selector to allow building specific group forum search index.
In the file ~\CMSModules\Forums\FormControls\ForumSelector.ascx.cs
there's a where condition that restricts the offered group forums:
// Select non group forum of current site
uniSelector.WhereCondition = ...
and it's done by this part of the condition:
... GroupGroupID IS NULL ...
so after removing this part, you would be able to select particularly the group forum(s).
This approach was not tested (goes beyond specification) are there are still some questions e.g. the need to avoid searching in group forum by a user who doesn't have access to that group / group forum...
Maybe if you could describe your group forums setup and your aim with search in group forums, it could help us to advise more accurately.
Regards,
ZdenekC