Copy paste from forum post:
Allow API access to group editing mode would be extremely helpful. We are an architecture firm, we use model groups heavily so lots of our work flow depends on it. (We used to work with lots of links but that didn't work well) Whenever there is a automation demand from users, it probably involves model group, and quite often I am hesitant to promise them anything because I know the ungroup-regroup workround is cumbersome and, sometimes, unstable. For us, this is one of the few pain points within Revit API that is really discouraging. I understand that getting API inside group editing mode might not be an easy task for the API team, since other "editing" modes are also missing from Revit API, but still, this is one of the most used functions in the whole software (needn't say an AutoCAD block analogy..), so lots of eager eyes are on it.
About the "unstableness" of ungroup-regroup approach: the model behaves not exactly in the way as when editing a group from within. One example is that some child element on a host could trigger the "ungoup or create new group" dialog during ungroup-regroup, while editing group from UI won't.